Use correct color values in quad fragment shader (#3217)

Previously, when using rounded corners, the borders would get wider.
Looks like we weren't using the input color values consistently in the
fragment shader. The quad values are raw HSLA and are converted in the
vertex shader.

Release Notes:

- N/A

crates/gpui2/src/platform/mac/shaders.metal

@@ -98,10 +98,10 @@ fragment float4 quad_fragment(QuadVertexOutput input [[stage_in]],
     input.border_color.a *= 1. - saturate(0.5 - inset_distance);
 
     // Alpha-blend the border and the background.
-    float output_alpha =
-        quad.border_color.a + quad.background.a * (1. - quad.border_color.a);
+    float output_alpha = input.border_color.a +
+                         input.background_color.a * (1. - input.border_color.a);
     float3 premultiplied_border_rgb =
-        input.border_color.rgb * quad.border_color.a;
+        input.border_color.rgb * input.border_color.a;
     float3 premultiplied_background_rgb =
         input.background_color.rgb * input.background_color.a;
     float3 premultiplied_output_rgb =