Add helper function to determine if color is transparent (#10310)

# Objective

- We need to check multiple times if a color is fully transparent, e.g.
for performance optimizations.
- Make code more readable.
- Reduce code duplication, to simplify making changes if needed (e.g. if
we need to take floating point weirdness into account later on).

## Solution

- Introduce a new `Color::is_fully_transparent` helper function to
determine if the alpha of a color is 0.
- Use the helper function in our UI rendering code.

---

## Changelog

- Added `Color::is_fully_transparent` helper function.

---------

Co-authored-by: François <mockersf@gmail.com>

crates/bevy_render/src/color/mod.rs

@@ -549,6 +549,25 @@ impl Color {
         self
     }
 
+    /// Determine if the color is fully transparent, i.e. if the alpha is 0.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use bevy_render::color::Color;
+    /// // Fully transparent colors
+    /// assert!(Color::NONE.is_fully_transparent());
+    /// assert!(Color::rgba(1.0, 0.5, 0.5, 0.0).is_fully_transparent());
+    ///
+    /// // (Partially) opaque colors
+    /// assert!(!Color::BLACK.is_fully_transparent());
+    /// assert!(!Color::rgba(1.0, 0.5, 0.5, 0.2).is_fully_transparent());
+    /// ```
+    #[inline(always)]
+    pub fn is_fully_transparent(&self) -> bool {
+        self.a() == 0.0
+    }
+
     /// Converts a `Color` to variant `Color::Rgba`
     pub fn as_rgba(self: &Color) -> Color {
         match self {

crates/bevy_ui/src/render/mod.rs

@@ -202,7 +202,7 @@ pub fn extract_atlas_uinodes(
         )) = uinode_query.get(*entity)
         {
             // Skip invisible and completely transparent nodes
-            if !view_visibility.get() || color.0.a() == 0.0 {
+            if !view_visibility.get() || color.0.is_fully_transparent() {
                 continue;
             }
 
@@ -306,7 +306,7 @@ pub fn extract_uinode_borders(
         {
             // Skip invisible borders
             if !view_visibility.get()
-                || border_color.0.a() == 0.0
+                || border_color.0.is_fully_transparent()
                 || node.size().x <= 0.
                 || node.size().y <= 0.
             {
@@ -413,7 +413,10 @@ pub fn extract_uinode_outlines(
             uinode_query.get(*entity)
         {
             // Skip invisible outlines
-            if !view_visibility.get() || outline.color.a() == 0. || node.outline_width == 0. {
+            if !view_visibility.get()
+                || outline.color.is_fully_transparent()
+                || node.outline_width == 0.
+            {
                 continue;
             }
 
@@ -508,7 +511,7 @@ pub fn extract_uinodes(
             uinode_query.get(*entity)
         {
             // Skip invisible and completely transparent nodes
-            if !view_visibility.get() || color.0.a() == 0.0 {
+            if !view_visibility.get() || color.0.is_fully_transparent() {
                 continue;
             }