Revert "gpui: Improve path rendering & global multisample anti-aliasing" (#34722)

Reverts zed-industries/zed#29718

We've noticed some issues with Zed on Intel-based Macs where typing has
become sluggish, and git bisect has seemed to point towards this PR.
Reverting for now, until we can understand why it is causing this issue.

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

@@ -698,27 +698,76 @@ fragment float4 polychrome_sprite_fragment(
   return color;
 }
 
-struct PathVertexOutput {
+struct PathRasterizationVertexOutput {
   float4 position [[position]];
+  float2 st_position;
+  float clip_rect_distance [[clip_distance]][4];
+};
+
+struct PathRasterizationFragmentInput {
+  float4 position [[position]];
+  float2 st_position;
+};
+
+vertex PathRasterizationVertexOutput path_rasterization_vertex(
+    uint vertex_id [[vertex_id]],
+    constant PathVertex_ScaledPixels *vertices
+    [[buffer(PathRasterizationInputIndex_Vertices)]],
+    constant Size_DevicePixels *atlas_size
+    [[buffer(PathRasterizationInputIndex_AtlasTextureSize)]]) {
+  PathVertex_ScaledPixels v = vertices[vertex_id];
+  float2 vertex_position = float2(v.xy_position.x, v.xy_position.y);
+  float2 viewport_size = float2(atlas_size->width, atlas_size->height);
+  return PathRasterizationVertexOutput{
+      float4(vertex_position / viewport_size * float2(2., -2.) +
+                 float2(-1., 1.),
+             0., 1.),
+      float2(v.st_position.x, v.st_position.y),
+      {v.xy_position.x - v.content_mask.bounds.origin.x,
+       v.content_mask.bounds.origin.x + v.content_mask.bounds.size.width -
+           v.xy_position.x,
+       v.xy_position.y - v.content_mask.bounds.origin.y,
+       v.content_mask.bounds.origin.y + v.content_mask.bounds.size.height -
+           v.xy_position.y}};
+}
+
+fragment float4 path_rasterization_fragment(PathRasterizationFragmentInput input
+                                            [[stage_in]]) {
+  float2 dx = dfdx(input.st_position);
+  float2 dy = dfdy(input.st_position);
+  float2 gradient = float2((2. * input.st_position.x) * dx.x - dx.y,
+                           (2. * input.st_position.x) * dy.x - dy.y);
+  float f = (input.st_position.x * input.st_position.x) - input.st_position.y;
+  float distance = f / length(gradient);
+  float alpha = saturate(0.5 - distance);
+  return float4(alpha, 0., 0., 1.);
+}
+
+struct PathSpriteVertexOutput {
+  float4 position [[position]];
+  float2 tile_position;
   uint sprite_id [[flat]];
   float4 solid_color [[flat]];
   float4 color0 [[flat]];
   float4 color1 [[flat]];
-  float4 clip_distance;
 };
 
-vertex PathVertexOutput path_vertex(
-    uint vertex_id [[vertex_id]],
-    constant PathVertex_ScaledPixels *vertices [[buffer(PathInputIndex_Vertices)]],
-    uint sprite_id [[instance_id]],
-    constant PathSprite *sprites [[buffer(PathInputIndex_Sprites)]],
-    constant Size_DevicePixels *input_viewport_size [[buffer(PathInputIndex_ViewportSize)]]) {
-  PathVertex_ScaledPixels v = vertices[vertex_id];
-  float2 vertex_position = float2(v.xy_position.x, v.xy_position.y);
-  float2 viewport_size = float2((float)input_viewport_size->width,
-                                (float)input_viewport_size->height);
+vertex PathSpriteVertexOutput path_sprite_vertex(
+    uint unit_vertex_id [[vertex_id]], uint sprite_id [[instance_id]],
+    constant float2 *unit_vertices [[buffer(SpriteInputIndex_Vertices)]],
+    constant PathSprite *sprites [[buffer(SpriteInputIndex_Sprites)]],
+    constant Size_DevicePixels *viewport_size
+    [[buffer(SpriteInputIndex_ViewportSize)]],
+    constant Size_DevicePixels *atlas_size
+    [[buffer(SpriteInputIndex_AtlasTextureSize)]]) {
+
+  float2 unit_vertex = unit_vertices[unit_vertex_id];
   PathSprite sprite = sprites[sprite_id];
-  float4 device_position = float4(vertex_position / viewport_size * float2(2., -2.) + float2(-1., 1.), 0., 1.);
+  // Don't apply content mask because it was already accounted for when
+  // rasterizing the path.
+  float4 device_position =
+      to_device_position(unit_vertex, sprite.bounds, viewport_size);
+  float2 tile_position = to_tile_position(unit_vertex, sprite.tile, atlas_size);
 
   GradientColor gradient = prepare_fill_color(
     sprite.color.tag,
@@ -728,32 +777,30 @@ vertex PathVertexOutput path_vertex(
     sprite.color.colors[1].color
   );
 
-  return PathVertexOutput{
+  return PathSpriteVertexOutput{
     device_position,
+    tile_position,
     sprite_id,
     gradient.solid,
     gradient.color0,
-    gradient.color1,
-    {v.xy_position.x - v.content_mask.bounds.origin.x,
-       v.content_mask.bounds.origin.x + v.content_mask.bounds.size.width -
-           v.xy_position.x,
-       v.xy_position.y - v.content_mask.bounds.origin.y,
-       v.content_mask.bounds.origin.y + v.content_mask.bounds.size.height -
-           v.xy_position.y}
+    gradient.color1
   };
 }
 
-fragment float4 path_fragment(
-    PathVertexOutput input [[stage_in]],
-    constant PathSprite *sprites [[buffer(PathInputIndex_Sprites)]]) {
-  if (any(input.clip_distance < float4(0.0))) {
-    return float4(0.0);
-  }
-
+fragment float4 path_sprite_fragment(
+    PathSpriteVertexOutput input [[stage_in]],
+    constant PathSprite *sprites [[buffer(SpriteInputIndex_Sprites)]],
+    texture2d<float> atlas_texture [[texture(SpriteInputIndex_AtlasTexture)]]) {
+  constexpr sampler atlas_texture_sampler(mag_filter::linear,
+                                          min_filter::linear);
+  float4 sample =
+      atlas_texture.sample(atlas_texture_sampler, input.tile_position);
+  float mask = 1. - abs(1. - fmod(sample.r, 2.));
   PathSprite sprite = sprites[input.sprite_id];
   Background background = sprite.color;
   float4 color = fill_color(background, input.position.xy, sprite.bounds,
     input.solid_color, input.color0, input.color1);
+  color.a *= mask;
   return color;
 }