vulkan: Repurpose debug flags for image uploads

vulkan: Repurpose debug flags for image uploads

Now that we don't use the old environment variables anymore to force
staging buffer/image uploads, we don't need them.

However, we do autodetect the fast path for avoiding a staging buffer
now, and we might want to be able to turn that off for testing.

So add GSK_DEBUG=staging that does exactly that.