clutter/stage: Only queue compressible events

Incompressible events already pass through unmodified, so queuing them just wasted time and memory.

We would however like to keep the ordering of events so we can only apply this optimization if the queue is empty.

This reduces the input latency of incompressible events like touchpad scrolling or drawing tablets by up to one frame. It also means the same series of events now arrives at the client more smoothly and not in bursts.