gpui: Improve path rendering & global multisample anti-aliasing (#29718)

Currently, the rendering path required creating a texture for each path,
which wasted a large amount of video memory. In our application, simply
drawing some charts resulted in video memory usage as high as 5G.

I removed the step of creating path textures and directly drew the paths
on the rendering target, adding post-processing global multi-sampling
anti-aliasing. Drawing paths no longer requires allocating any
additional video memory and also improves the performance of path
rendering.

Release Notes:

- N/A

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>

crates/gpui/src/platform/test/window.rs

@@ -341,7 +341,7 @@ impl PlatformAtlas for TestAtlas {
             crate::AtlasTile {
                 texture_id: AtlasTextureId {
                     index: texture_id,
-                    kind: crate::AtlasTextureKind::Path,
+                    kind: crate::AtlasTextureKind::Polychrome,
                 },
                 tile_id: TileId(tile_id),
                 padding: 0,