[Direct3D12] Have `D3D12_RENDER_TARGET_BLEND_DESC::RenderTargetWriteMask` be of type `D3D12_COLOR_WRITE_ENABLE`

### Suggestion

Currently `D3D12_RENDER_TARGET_BLEND_DESC::RenderTargetWriteMask` is of type `u8`.
But the masks are `D3D12_COLOR_WRITE_ENABLE`.
So to use them one must do this:
```rust
D3D12_RENDER_TARGET_BLEND_DESC {
RenderTargetWriteMask: D3D12_COLOR_WRITE_ENABLE_ALL.0 as u8,
..Default::default()
}
```

Trace how D3D12_RENDER_TARGET_BLEND_DESC and D3D12_COLOR_WRITE_ENABLE are represented in the metadata and generated Rust bindings. Confirm the generated RenderTargetWriteMask type matches D3D12_COLOR_WRITE_ENABLE and verify the existing generation or validation checks; done means consumers no longer need the u8 cast shown in the issue.

Written by the indexing model from the issue text.