UI materials: don't reserve in loop when enough capacity (#15919)

# Objective

- UI materials reserve too much capacity in a vec: for every node in the
transparent phase, it reserves enough memory to store all the nodes
- Update #10437 

## Solution

- Only reserve extra memory if there's not enough
- Only reserve the needed memory, not more

crates/bevy_ui/src/render/ui_material_pipeline.rs

@@ -642,9 +642,11 @@ pub fn queue_ui_material_nodes<M: UiMaterial>(
                 bind_group_data: material.key.clone(),
             },
         );
-        transparent_phase
-            .items
-            .reserve(extracted_uinodes.uinodes.len());
+        if transparent_phase.items.capacity() < extracted_uinodes.uinodes.len() {
+            transparent_phase.items.reserve_exact(
+                extracted_uinodes.uinodes.len() - transparent_phase.items.capacity(),
+            );
+        }
         transparent_phase.add(TransparentUi {
             draw_function,
             pipeline,