Fix unbatchable meshes. (#18761)

# Objective

Fixes #18550.

Because bin state for unbatchable meshes wasn't being cleared each
frame, the buffer indices for unbatchable meshes would demote from
sparse to dense storage and aggressively leak memory, with all kinds of
weird consequences downstream, namely supplying invalid instance ranges
for render.

## Solution

Clear out the unbatchable mesh bin state when we start a new frame.

crates/bevy_render/src/render_phase/mod.rs

@@ -67,6 +67,7 @@ use bevy_ecs::{
 };
 use core::{fmt::Debug, hash::Hash, iter, marker::PhantomData, ops::Range, slice::SliceIndex};
 use smallvec::SmallVec;
+use tracing::warn;
 
 /// Stores the rendering instructions for a single phase that uses bins in all
 /// views.
@@ -853,6 +854,10 @@ where
             .set_range(self.cached_entity_bin_keys.len().., true);
 
         self.entities_that_changed_bins.clear();
+
+        for unbatchable_bin in self.unbatchable_meshes.values_mut() {
+            unbatchable_bin.buffer_indices.clear();
+        }
     }
 
     /// Checks to see whether the entity is in a bin and returns true if it's
@@ -1325,6 +1330,10 @@ impl UnbatchableBinnedEntityIndexSet {
                 // but let's go ahead and do the sensible thing anyhow: demote
                 // the compressed `NoDynamicOffsets` field to the full
                 // `DynamicOffsets` array.
+                warn!(
+                    "Unbatchable binned entity index set was demoted from sparse to dense. \
+                    This is a bug in the renderer. Please report it.",
+                );
                 let new_dynamic_offsets = (0..instance_range.len() as u32)
                     .flat_map(|entity_index| self.indices_for_entity_index(entity_index))
                     .chain(iter::once(indices))
@@ -1337,6 +1346,17 @@ impl UnbatchableBinnedEntityIndexSet {
             }
         }
     }
+
+    /// Clears the unbatchable binned entity index set.
+    fn clear(&mut self) {
+        match self {
+            UnbatchableBinnedEntityIndexSet::Dense(dense_indices) => dense_indices.clear(),
+            UnbatchableBinnedEntityIndexSet::Sparse { .. } => {
+                *self = UnbatchableBinnedEntityIndexSet::NoEntities;
+            }
+            _ => {}
+        }
+    }
 }
 
 /// A collection of all items to be rendered that will be encoded to GPU