waylandsink: Update stale GstBuffer references in wayland buffer cache

"waylandsink: use GstMemory instead of GstBuffer for cache lookup" changes the cache key to GstMemory, but the cached data still needs a pointer to the GstBuffer to control the buffer lifecycle. If the GstMemory used as the cache key moves from one GstBuffer to another, the pointer in the cached data will be out-of-date.

Update the current GstBuffer pointer for each frame so that it always represents the currently in use (from attach to release) GstBuffer for each wl_buffer.