master-clock: Check for CLUTTER_FEATURE_SWAP_EVENTS
Because the absence of `CLUTTER_FEATURE_SYNC_TO_VBLANK` alone isn't reason to drop down to the 60Hz fallback. The very previous comment already states "or swap-buffer-complete events if supported in the backend". And the code immediately above that actually does the syncing to swap events via `master_clock_get_swap_wait_time`. So if we have `CLUTTER_FEATURE_SWAP_EVENTS` but not `CLUTTER_FEATURE_SYNC_TO_VBLANK` then we still want to return immediately, avoiding getting rate limited to 60Hz.
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