radv: add external memory support.

radv: add external memory support.

This adds support for exporting 2D images, to an
opaque fd.

This implements the:
VK_KHX_external_memory_capabilities
VK_KHX_external_memory
VK_KHX_external_memory_fd

extensions.

These are used by SteamVR, we should work with anv
to decide if we should ship these under an env
var or something.

v2 (Bas): - Don't expose the semaphore ext without implementing it.
          - Only export the capabilities ext as instance ext.
          - Implement radv_GetPhysicalDeviceExternalBufferPropertiesKHX.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Bas Nieuwenhuizen <basni@google.com>