Only regenerate text measure funcs on changes to the scale factor, not the camera or target size (#20538)

# Objective

`measure_text_system` should only remeasure text nodes on changes to
scale factor and their text, not the target camera or target size.

## Solution

Instead of checking if the `ComputedUiTargetCamera` component has
changed, compare the computed target scale factor to the scale factor
from the previous frame stored in `ComputedNode`.

Author: ickshonpe

crates/bevy_ui/src/lib.rs

@@ -188,6 +188,7 @@ impl Plugin for UiPlugin {
                 ui_stack_system
                     .in_set(UiSystems::Stack)
                     // These systems don't care about stack index
+                    .ambiguous_with(widget::measure_text_system)
                     .ambiguous_with(update_clipping_system)
                     .ambiguous_with(ui_layout_system)
                     .ambiguous_with(widget::update_viewport_render_target_size)

crates/bevy_ui/src/widget/text.rs

@@ -276,17 +276,22 @@ pub fn measure_text_system(
             &mut ContentSize,
             &mut TextNodeFlags,
             &mut ComputedTextBlock,
-            Ref<ComputedUiTargetCamera>,
+            &ComputedUiTargetCamera,
+            &ComputedNode,
         ),
         With<Node>,
     >,
     mut text_reader: TextUiReader,
     mut text_pipeline: ResMut<TextPipeline>,
     mut font_system: ResMut<CosmicFontSystem>,
 ) {
-    for (entity, block, content_size, text_flags, computed, computed_target) in &mut text_query {
+    for (entity, block, content_size, text_flags, computed, computed_target, computed_node) in
+        &mut text_query
+    {
         // Note: the ComputedTextBlock::needs_rerender bool is cleared in create_text_measure().
-        if computed_target.is_changed()
+        // 1e-5 epsilon to ignore tiny scale factor float errors
+        if 1e-5
+            < (computed_target.scale_factor() - computed_node.inverse_scale_factor.recip()).abs()
             || computed.needs_rerender()
             || text_flags.needs_measure_fn
             || content_size.is_added()