u/sink: Make sink_queue drop the oldest frame when full

Instead of discarding the incoming frame when the queue is
at capacity, drop the oldest frame possible. I can't
think of a situation where it would be better to work on
stale data in preference to fresh data. If there is, we
can easily add a flag to choose behaviour.

Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2188>

src/xrt/auxiliary/util/u_sink_queue.c

···
 queue_try_refpush(struct u_sink_queue *q, struct xrt_frame *xf)
 {
 	if (queue_is_full(q)) {
-		return false;
+		// Drop the oldest frame
+		if (!queue_is_empty(q)) {
+			struct xrt_frame *old = queue_pop(q);
+			xrt_frame_reference(&old, NULL);
+		}
 	}
 	struct u_sink_queue_elem *elem = U_TYPED_CALLOC(struct u_sink_queue_elem);
 	xrt_frame_reference(&elem->frame, xf);