Fix the UV calculations for clipped and flipped ImageNodes (#8195)

# Objective

Instead of flipping the entire image, `prepare_ui_nodes` only flips the
unclipped area.

<img width="652" alt="overflow_flipped_bug"
src="https://user-images.githubusercontent.com/27962798/227587867-1467c6ae-8693-45c3-87cb-793cc5b433e4.png">

## Solution

Whenever flip_x or flip_y is set swap the image rect coordinates and
invert the clipping coords along the flipped axes before the UVs are
calculated.

<img width="656" alt="overflow_fixed"
src="https://user-images.githubusercontent.com/27962798/227588839-e0dde3b9-dc26-4652-a129-2faab2d07281.PNG">

--

## Changelog

* Modified `prepare_uinodes` so that the UVs for clipped and flipped
image nodes are calculated correctly.

crates/bevy_ui/src/render/mod.rs

@@ -414,7 +414,8 @@ pub fn prepare_uinodes(
             current_batch_handle = extracted_uinode.image.clone_weak();
         }
 
-        let uinode_rect = extracted_uinode.rect;
+        let mut uinode_rect = extracted_uinode.rect;
+
         let rect_size = uinode_rect.size().extend(1.0);
 
         // Specify the corners of the node
@@ -423,7 +424,7 @@ pub fn prepare_uinodes(
 
         // Calculate the effect of clipping
         // Note: this won't work with rotation/scaling, but that's much more complex (may need more that 2 quads)
-        let positions_diff = if let Some(clip) = extracted_uinode.clip {
+        let mut positions_diff = if let Some(clip) = extracted_uinode.clip {
             [
                 Vec2::new(
                     f32::max(clip.min.x - positions[0].x, 0.),
@@ -473,7 +474,21 @@ pub fn prepare_uinodes(
             [Vec2::ZERO, Vec2::X, Vec2::ONE, Vec2::Y]
         } else {
             let atlas_extent = extracted_uinode.atlas_size.unwrap_or(uinode_rect.max);
-            let mut uvs = [
+            if extracted_uinode.flip_x {
+                std::mem::swap(&mut uinode_rect.max.x, &mut uinode_rect.min.x);
+                positions_diff[0].x *= -1.;
+                positions_diff[1].x *= -1.;
+                positions_diff[2].x *= -1.;
+                positions_diff[3].x *= -1.;
+            }
+            if extracted_uinode.flip_y {
+                std::mem::swap(&mut uinode_rect.max.y, &mut uinode_rect.min.y);
+                positions_diff[0].y *= -1.;
+                positions_diff[1].y *= -1.;
+                positions_diff[2].y *= -1.;
+                positions_diff[3].y *= -1.;
+            }
+            [
                 Vec2::new(
                     uinode_rect.min.x + positions_diff[0].x,
                     uinode_rect.min.y + positions_diff[0].y,
@@ -491,15 +506,7 @@ pub fn prepare_uinodes(
                     uinode_rect.max.y + positions_diff[3].y,
                 ),
             ]
-            .map(|pos| pos / atlas_extent);
-
-            if extracted_uinode.flip_x {
-                uvs = [uvs[1], uvs[0], uvs[3], uvs[2]];
-            }
-            if extracted_uinode.flip_y {
-                uvs = [uvs[3], uvs[2], uvs[1], uvs[0]];
-            }
-            uvs
+            .map(|pos| pos / atlas_extent)
         };
 
         let color = extracted_uinode.color.as_linear_rgba_f32();