Shrink ComputedVisibility (#6305)

# Objective
`ComputedVisibility` could afford to be smaller/faster. Optimizing the size and performance of operations on the component will positively benefit almost all extraction systems.

This was listed as one of the potential pieces of future work for #5310.

## Solution
Merge both internal booleans into a single `u8` bitflag field. Rely on bitmasks to evaluate local, hierarchical, and general visibility.

Pros:

 - `ComputedVisibility::is_visible` should be a single bitmask test instead of two.
 - `ComputedVisibility` is now only 1 byte. Should be able to fit 100% more per cache line when using dense iteration.

Cons:

 - Harder to read.
 - Setting individual values inside `ComputedVisiblity` require bitmask mutations. 

This should be a non-breaking change. No public API was changed. The only publicly visible effect is that `ComputedVisibility` is now 1 byte instead of 2.

crates/bevy_render/src/view/visibility/mod.rs

@@ -54,12 +54,19 @@ impl Visibility {
     }
 }
 
+bitflags::bitflags! {
+    #[derive(Reflect)]
+    struct ComputedVisibilityFlags: u8 {
+        const VISIBLE_IN_VIEW = 1 << 0;
+        const VISIBLE_IN_HIERARCHY = 1 << 1;
+    }
+}
+
 /// Algorithmically-computed indication of whether an entity is visible and should be extracted for rendering
 #[derive(Component, Clone, Reflect, Debug, Eq, PartialEq)]
 #[reflect(Component, Default)]
 pub struct ComputedVisibility {
-    is_visible_in_hierarchy: bool,
+    flags: ComputedVisibilityFlags,
-    is_visible_in_view: bool,
 }
 
 impl Default for ComputedVisibility {
@@ -71,8 +78,7 @@ impl Default for ComputedVisibility {
 impl ComputedVisibility {
     /// A [`ComputedVisibility`], set as invisible.
     pub const INVISIBLE: Self = ComputedVisibility {
-        is_visible_in_hierarchy: false,
+        flags: ComputedVisibilityFlags::empty(),
-        is_visible_in_view: false,
     };
 
     /// Whether this entity is visible to something this frame. This is true if and only if [`Self::is_visible_in_hierarchy`] and [`Self::is_visible_in_view`]
@@ -81,7 +87,7 @@ impl ComputedVisibility {
     /// [`CoreStage::Update`] stage will yield the value from the previous frame.
     #[inline]
     pub fn is_visible(&self) -> bool {
-        self.is_visible_in_hierarchy && self.is_visible_in_view
+        self.flags.bits == ComputedVisibilityFlags::all().bits
     }
 
     /// Whether this entity is visible in the entity hierarchy, which is determined by the [`Visibility`] component.
@@ -90,7 +96,8 @@ impl ComputedVisibility {
     /// [`VisibilitySystems::VisibilityPropagate`] system label.
     #[inline]
     pub fn is_visible_in_hierarchy(&self) -> bool {
-        self.is_visible_in_hierarchy
+        self.flags
+            .contains(ComputedVisibilityFlags::VISIBLE_IN_HIERARCHY)
     }
 
     /// Whether this entity is visible in _any_ view (Cameras, Lights, etc). Each entity type (and view type) should choose how to set this
@@ -102,7 +109,8 @@ impl ComputedVisibility {
     /// Other entities might just set this to `true` every frame.
     #[inline]
     pub fn is_visible_in_view(&self) -> bool {
-        self.is_visible_in_view
+        self.flags
+            .contains(ComputedVisibilityFlags::VISIBLE_IN_VIEW)
     }
 
     /// Sets `is_visible_in_view` to `true`. This is not reversible for a given frame, as it encodes whether or not this is visible in
@@ -111,7 +119,16 @@ impl ComputedVisibility {
     /// label. Don't call this unless you are defining a custom visibility system. For normal user-defined entity visibility, see [`Visibility`].
     #[inline]
     pub fn set_visible_in_view(&mut self) {
-        self.is_visible_in_view = true;
+        self.flags.insert(ComputedVisibilityFlags::VISIBLE_IN_VIEW);
+    }
+
+    #[inline]
+    fn reset(&mut self, visible_in_hierarchy: bool) {
+        self.flags = if visible_in_hierarchy {
+            ComputedVisibilityFlags::VISIBLE_IN_HIERARCHY
+        } else {
+            ComputedVisibilityFlags::empty()
+        };
     }
 }
 
@@ -280,13 +297,12 @@ fn visibility_propagate_system(
     children_query: Query<&Children, (With<Parent>, With<Visibility>, With<ComputedVisibility>)>,
 ) {
     for (children, visibility, mut computed_visibility, entity) in root_query.iter_mut() {
-        computed_visibility.is_visible_in_hierarchy = visibility.is_visible;
         // reset "view" visibility here ... if this entity should be drawn a future system should set this to true
-        computed_visibility.is_visible_in_view = false;
+        computed_visibility.reset(visibility.is_visible);
         if let Some(children) = children {
             for child in children.iter() {
                 let _ = propagate_recursive(
-                    computed_visibility.is_visible_in_hierarchy,
+                    computed_visibility.is_visible_in_hierarchy(),
                     &mut visibility_query,
                     &children_query,
                     *child,
@@ -313,10 +329,10 @@ fn propagate_recursive(
             child_parent.get(), expected_parent,
             "Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
         );
-        computed_visibility.is_visible_in_hierarchy = visibility.is_visible && parent_visible;
+        let visible_in_hierarchy = visibility.is_visible && parent_visible;
         // reset "view" visibility here ... if this entity should be drawn a future system should set this to true
-        computed_visibility.is_visible_in_view = false;
+        computed_visibility.reset(visible_in_hierarchy);
-        computed_visibility.is_visible_in_hierarchy
+        visible_in_hierarchy
     };
 
     for child in children_query.get(entity).map_err(drop)?.iter() {
@@ -390,7 +406,7 @@ pub fn check_visibility(
                     }
                 }
 
-                computed_visibility.is_visible_in_view = true;
+                computed_visibility.set_visible_in_view();
                 let cell = thread_queues.get_or_default();
                 let mut queue = cell.take();
                 queue.push(entity);
@@ -412,7 +428,7 @@ pub fn check_visibility(
                     return;
                 }
 
-                computed_visibility.is_visible_in_view = true;
+                computed_visibility.set_visible_in_view();
                 let cell = thread_queues.get_or_default();
                 let mut queue = cell.take();
                 queue.push(entity);
@@ -518,7 +534,7 @@ mod test {
                 .entity(e)
                 .get::<ComputedVisibility>()
                 .unwrap()
-                .is_visible_in_hierarchy
+                .is_visible_in_hierarchy()
         };
         assert!(
             !is_visible(root1),