clutter/frame-clock: Start next update ASAP after idle period

For frame updates in response to sporadic user interaction, this results in input → output latency somewhere between the minimum possible and the minimum plus the length of one display refresh cycle (assuming the frame update can complete within a refresh cycle).

Applying a max_render_time based deadline which corresponds to higher than the minimum possible latency would result in higher effective minimum latency for sporadic user interaction.

This was discovered by @YaLTeR, based on measurements described in https://mastodon.online/@YaLTeR/110848066454900941 .

Plus fix up a couple of tests to still pass with this change, and a minor simplification.