text aspect ratio bug fix (bevyengine#6825)

## Objective 

Bevy UI uses a `MeasureFunc` that preserves the aspect ratio of text, not just images. This means that the extent of flex-items containing text may be calculated incorrectly depending on the ratio of the text size compared to the size of its containing node.

Fixes bevyengine#6748 
Related to bevyengine#6724

with Bevy 0.9:

![Capture_cols_0 9](https://user-images.githubusercontent.com/27962798/205435999-386d3400-fe9b-475a-aab1-18e61c4c074f.PNG)

with this PR (accurately matching the behavior of Flexbox):

![Capture_fixed](https://user-images.githubusercontent.com/27962798/205436005-6bafbcc2-cd87-4eb7-b5c6-9dbcb30fc795.PNG)

## Solution
Only perform the aspect ratio calculations if the uinode contains an image.

## Changelog
* Added a field `preserve_aspect_ratio` to `CalculatedSize`
* The `MeasureFunc` only preserves the aspect ratio when `preserve_aspect_ratio` is true.
* `update_image_calculated_size_system` sets `preserve_aspect_ratio` to true for nodes with images.

crates/bevy_ui/src/flex/mod.rs

@@ -86,11 +86,15 @@ impl FlexSurface {
                 match (constraints.width, constraints.height) {
                     (Number::Undefined, Number::Undefined) => {}
                     (Number::Defined(width), Number::Undefined) => {
-                        size.height = width * size.height / size.width;
+                        if calculated_size.preserve_aspect_ratio {
+                            size.height = width * size.height / size.width;
+                        }
                         size.width = width;
                     }
                     (Number::Undefined, Number::Defined(height)) => {
-                        size.width = height * size.width / size.height;
+                        if calculated_size.preserve_aspect_ratio {
+                            size.width = height * size.width / size.height;
+                        }
                         size.height = height;
                     }
                     (Number::Defined(width), Number::Defined(height)) => {
@@ -236,12 +240,6 @@ pub fn flex_node_system(
     let logical_to_physical_factor = windows.scale_factor(WindowId::primary());
     let scale_factor = logical_to_physical_factor * ui_scale.scale;
 
-    if scale_factor_events.iter().next_back().is_some() || ui_scale.is_changed() {
-        update_changed(&mut flex_surface, scale_factor, full_node_query);
-    } else {
-        update_changed(&mut flex_surface, scale_factor, node_query);
-    }
-
     fn update_changed<F: ReadOnlyWorldQuery>(
         flex_surface: &mut FlexSurface,
         scaling_factor: f64,
@@ -258,6 +256,12 @@ pub fn flex_node_system(
         }
     }
 
+    if scale_factor_events.iter().next_back().is_some() || ui_scale.is_changed() {
+        update_changed(&mut flex_surface, scale_factor, full_node_query);
+    } else {
+        update_changed(&mut flex_surface, scale_factor, node_query);
+    }
+
     for (entity, style, calculated_size) in &changed_size_query {
         flex_surface.upsert_leaf(entity, style, *calculated_size, scale_factor);
     }

crates/bevy_ui/src/ui_node.rs

@@ -434,6 +434,8 @@ pub enum FlexWrap {
 pub struct CalculatedSize {
     /// The size of the node
     pub size: Size,
+    /// Whether to attempt to preserve the aspect ratio when determing the layout for this item
+    pub preserve_aspect_ratio: bool,
 }
 
 /// The background color of the node

crates/bevy_ui/src/widget/image.rs

@@ -21,6 +21,7 @@ pub fn update_image_calculated_size_system(
             // Update only if size has changed to avoid needless layout calculations
             if size != calculated_size.size {
                 calculated_size.size = size;
+                calculated_size.preserve_aspect_ratio = true;
             }
         }
     }