Fix UI border artifacts caused by incorrect blending (#12725)

Fixes #12702.

Co-authored-by: François Mockers <mockersf@gmail.com>

crates/bevy_ui/src/render/ui.wgsl

@@ -159,12 +159,15 @@ fn draw(in: VertexOutput) -> vec4<f32> {
         // is present, otherwise an outline about the external boundary would be drawn even without 
         // a border.
         let t = 1. - select(step(0.0, border_distance), smoothstep(0.0, fborder, border_distance), external_distance < internal_distance);
-        return color.rgba * t;
+
+        // Blend mode ALPHA_BLENDING is used for UI elements, so we don't premultiply alpha here.
+        return vec4(color.rgb, color.a * t);
     }
 
     // The item is a rectangle, draw normally with anti-aliasing at the edges.
     let t = 1. - smoothstep(0.0, fexternal, external_distance);
-    return color.rgba * t;
+
+    return vec4(color.rgb, color.a * t);
 }
 
 @fragment