gpui: Store GPU-facing bools as PaddedBool32 to avoid uninitialized padding (#60482)

`PolychromeSprite` in `crates/gpui/src/scene.rs` is `#[repr(C)]` and had
a `grayscale: bool` field followed by 3 compiler-inserted padding bytes
that were never written. The wgpu renderer's `instance_bytes`
reinterprets `&[PolychromeSprite]` as `&[u8]` via
`slice::from_raw_parts` and passes it to `queue.write_buffer`, so those
uninitialized padding bytes were exposed behind a shared `&[u8]` on
every frame that draws an image or emoji, which is undefined behavior.

Rather than widening the field to a raw `u32` (which would suggest
values other than 0 and 1 are meaningful), this introduces
`PaddedBool32`: a `#[repr(transparent)]` wrapper around `u32` whose only
public constructor is `From<bool>`, so the 0-or-1 invariant is enforced
by the type while the layout has no padding. `Underline.wavy`, which was
already a raw `u32` for the same reason, is converted too.

cbindgen emits the wrapper as `typedef uint32_t PaddedBool32;`, so the
generated Metal header and shaders are unchanged. The WGSL and HLSL
shaders already declared these fields as `u32`/`uint`; their `& 0xFFu`
masks, which existed to ignore the garbage padding bytes, are now
simplified to plain comparisons.

Release Notes:

- N/A

crates/gpui_windows/src/shaders.hlsl

@@ -1249,7 +1249,7 @@ float4 polychrome_sprite_fragment(PolychromeSpriteFragmentInput input): SV_Targe
     float distance = quad_sdf(input.position.xy, sprite.bounds, sprite.corner_radii);
 
     float4 color = sample;
-    if ((sprite.grayscale & 0xFFu) != 0u) {
+    if (sprite.grayscale != 0u) {
         float3 grayscale = dot(color.rgb, GRAYSCALE_FACTORS);
         color = float4(grayscale, sample.a);
     }