Fix border rendering (#3884)

After implementing it a while ago, our previous interpolation scheme
didn't really make sense to me and was causing borders to be rendered
incorrectly. We don't really draw backgrounds and borders as part of the
same draw call anymore, but it seemed reasonable to have a correct
implementation in the shader anyway.

This commit uses Porter-Duff compositing (i.e., `over`) to produce a
color that is the result of superimposing the border on top of the
background. Then, we linearly interpolate towards the background color
as we slide out of the border and into the background.

Release Notes:

- Fixed incorrect rendering of borders.

crates/gpui/src/platform/mac/shaders.metal

@@ -16,6 +16,7 @@ float gaussian(float x, float sigma);
 float2 erf(float2 x);
 float blur_along_x(float x, float y, float sigma, float corner,
                    float2 half_size);
+float4 over(float4 below, float4 above);
 
 struct QuadVertexOutput {
   float4 position [[position]];
@@ -108,21 +109,11 @@ fragment float4 quad_fragment(QuadFragmentInput input [[stage_in]],
     color = input.background_color;
   } else {
     float inset_distance = distance + border_width;
-
-    // Decrease border's opacity as we move inside the background.
-    input.border_color.a *= 1. - saturate(0.5 - inset_distance);
-
-    // Alpha-blend the border and the background.
-    float output_alpha = input.border_color.a +
-                         input.background_color.a * (1. - input.border_color.a);
-    float3 premultiplied_border_rgb =
-        input.border_color.rgb * input.border_color.a;
-    float3 premultiplied_background_rgb =
-        input.background_color.rgb * input.background_color.a;
-    float3 premultiplied_output_rgb =
-        premultiplied_border_rgb +
-        premultiplied_background_rgb * (1. - input.border_color.a);
-    color = float4(premultiplied_output_rgb, output_alpha);
+    // Blend the border on top of the background and then linearly interpolate
+    // between the two as we slide inside the background.
+    float4 blended_border = over(input.background_color, input.border_color);
+    color = mix(blended_border, input.background_color,
+                saturate(0.5 - inset_distance));
   }
 
   return color * float4(1., 1., 1., saturate(0.5 - distance));
@@ -653,3 +644,12 @@ float4 distance_from_clip_rect(float2 unit_vertex, Bounds_ScaledPixels bounds,
                 position.y - clip_bounds.origin.y,
                 clip_bounds.origin.y + clip_bounds.size.height - position.y);
 }
+
+float4 over(float4 below, float4 above) {
+  float4 result;
+  float alpha = above.a + below.a * (1.0 - above.a);
+  result.rgb =
+      (above.rgb * above.a + below.rgb * below.a * (1.0 - above.a)) / alpha;
+  result.a = alpha;
+  return result;
+}