WIP: clutter-stage-cogl: Never cache the update_time [performance]

Previously we would calculate the stage update_time once per frame and keep returning the same value until it was cleared.

The problem with that was that by the time we came to render we were using the wrong update_time. It was out of date because it had been calculated from an old value of last_presentation_time and not the latest one. The effect of this was that we would briefly schedule frames faster than the refresh rate, filling the buffer queue. Once the queue was full we would be throttled to the correct refresh rate by SwapBuffers but the buffer queue would remain full (triple buffered) causing visible lag.

The solution is to not cache update_time, just recalculate it more often. This way as we race against the pending presentation event, after it completes we are able to recalculate the correct update_time before rendering. Thus avoiding rendering too early and avoiding filling triple buffers when we only want double buffering.

Closes: #334 (closed)