Manually copy across a shader fix from WR.

This fix has landed in WR, but WR can't be updated right now, due
to a scroll change landing in WR without the Servo PR being ready
quite yet.

Fixes #14061.
Fixes #14073.

resources/shaders/ps_image_clip.vs.glsl

@@ -27,14 +27,9 @@ void main(void) {
     write_clip(clip);
 
     // vUv will contain how many times this image has wrapped around the image size.
-    vec2 st0 = image.st_rect.xy;
-    vec2 st1 = image.st_rect.zw;
-
-    if (image.has_pixel_coords) {
-        vec2 texture_size = vec2(textureSize(sDiffuse, 0));
-        st0 /= texture_size;
-        st1 /= texture_size;
-    }
+    vec2 texture_size = vec2(textureSize(sDiffuse, 0));
+    vec2 st0 = image.st_rect.xy / texture_size;
+    vec2 st1 = image.st_rect.zw / texture_size;
 
     vTextureSize = st1 - st0;
     vTextureOffset = st0;