radv: implement VK_EXT_sample_locations and disable it

radv: implement VK_EXT_sample_locations and disable it

Basically, this extension allows applications to use custom
sample locations. It doesn't support variable sample locations
during subpass. Note that we don't have to upload the user
sample locations because the spec doesn't allow this.

The extension is currently disabled because the driver needs to
support variable sample locations during layout transitions. The
depth decompress needs to know them and that's a bit invasive.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>