Support client shm buffers with VSP2 hardware compositing

The current VSP2 Wayland compositing only supports buffers that can be acquired through wayland-kms.

As far as I know, wayland-kms doesn't work for shared memory buffers, so we need an alternative way to get a buffer handle that the VSP2 can use.

Weston uses the linux_dmabuf extension to do this, so this is one alternative.

Another alternative would be to try to convert the shared memory buffers into dma buffers, but I'm not sure how it can be done without hurting performance.