vrend: Support emulating planar image sampling
Mesa emulates planar format sampling with per-plane samplers. With this change, the guest can pass the plane index when creating a sampler view from a virgl resource to create a per-plane sampler view. The index can be passed in place of the texture layer, as that will always be 0 for planar images.
Signed-off-by: David Stevens stevensd@chromium.org