WIP: clutter-stage-cogl: Disable urgent frames when swap event support is missing
If CLUTTER_FEATURE_SWAP_EVENTS
isn't available then that means
pending_swaps
will always have value zero and we therefore can't rely
on clutter_stage_cogl_get_update_time
to enforce throttling to the
refresh rate. So we would get high latency as the swap buffers filled up
at an unboundedly fast rate. That's bad, so if the feature isn't
available just schedule slightly early -- still with zero sync_delay
but no longer faster than the refresh interval.
As requested/recommended by @otaylor