Non-ideal frame clock if swap events not supported.

As discussed in !1284 (closed), if swap events are not supported, then in clutter_stage_cogl_schedule_update the last_presentation_time is always zero. This means we take the next_presentation_time < now codepath, and hardware_clock_phase is always zero (because last_presentation_time is zero).

We will still pick some time that is synchronized with a cycle of refresh_interval, but since we don't know any presentation time we can use it can be up to one refresh_interval out of phase which is not ideal. It would be good to have a special codepath for last_presentation_time==0 that tried to estimate the hardware clock phase in some other way.