vulkan/wsi: Align buffer image strides to optimalBufferCopyRowPitchAlignment

This isn't a big deal for the current buffer paths because the required
alignment for PRIME is already higher than any driver advertises.
However, the SW path we're about to add won't have the PRIME requirement.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17388>