wgpu: use mailbox instead of fifo for vsync

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This has lower latency (for the platforms that support it). It will fall back to Fifo for platforms that don't support it. Hopefully this will resolve the "input lag" problem for most people.

Its worth pointing out that this doesn't actually resolve the lag problem for me on linux/gnome-x11/nvidia gtx 1070/proprietary driver. Its possible that some aspect of that combination doesn't support "mailbox".

With that setup (and a 100fps monitor) i get 3 frames of lag.

If i disable vsync entirely, i get 1 frame of lag.

This resolved the lag problem for @zachreizner, who suggested this change in the first place.

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@bjorn3 I'm curious if this fixes the lag problem for you (in part or in whole). If you find the time (and the motivation) to run another OBS test, let me know!