queue pending buffers for wayland shm clients
queue pending buffers for wayland shm clients

due to how deferred rendering works, it's possible for a client to
send a second buffer before enlightenment has rendered the first one.
in this situation, it seems that the best solution is to queue successive
buffers (frames) and pop the queue after each render

ref T2784

rvandegrift/e,rvandegrift/e,tasn/enlightenment,tizenorg/platform.upstream.enlightenment,tasn/enlightenment,tasn/enlightenment,tizenorg/platform.upstream.enlightenment,rvandegrift/e,tizenorg/platform.upstream.enlightenment