winevulkan: fshack: Fixes and improvements.

Old fshack causes radv assertions because of image view format mismatches.

Changes:
- drop the vkCmdBlitImage path, it's impossible to keep without
  VK_KHR_swapchain_mutable_format.
- drop the compute+copy path, all drivers support swapchains with storage usage,
  no reason to keep untested code around.
- interpolate in linear color space for slightly better results
- improve compute path by interpolating at the pixel center
- fix all validation errors

Signed-off-by: Georg Lehmann <[email protected]>
Link: #158