fill_buffer start has to be 16-byte aligned on DX11

[kvark]

See gfx-rs/wgpu#1303 (comment)
We may need to expose a limit for this.
cc @cwfitzgerald

[kvark]

@cwfitzgerald It occurred to me that DX11 backend can hide this limitation internally. It can basically create a 16-byte buffer with this data and issue a buffer-buffer copy for the unaligned part.

[cwfitzgerald]

I'd be quite worried about the performance of this. Maybe expose a "performance only" limit so that wgpu can always properly align it, and if the value is unaligned we can fall back to this workaround.

[kvark]

I mean, there is clearly a naive path here. We can do better by making a pool of, say 256 byte buffers that are re-used, in which case the major cost - allocating a new buffer - goes away. The point is that it's possible to implement, and similar workarounds are already employed in other places (unfortunately, I suppose?).

Maybe when we finally change the direction to #2249, we can remove the workarounds.