wgpu_hal::dx12 should not allocate a 256KiB buffer just to hold zeroes

[jimblandy]

In wgpu_hal, dx12::Device should not spend 256KiB of GPU memory on Device::zero_buffer, whose only reason for existence is to serve as the source of CopyBufferRegion calls in <dx12::Device as CommandEncoder>::clear_buffer.

It seems like the only other feasible way to zero a buffer in Direct3D 12 is with a compute shader.

[jimblandy]

Correction - it's only 256KiB, not 256MiB.

[magcius]

There's also https://learn.microsoft.com/en-us/windows/win32/api/d3d12/nf-d3d12-id3d12graphicscommandlist-clearunorderedaccessviewuint