Present mode / vsync / frame limiting

[dhardy]

wgpu::PresentMode supports Fifo which limits frames (effectively vsync); unfortunately this causes lag when resizing a window which I haven't found a different way to solve.

Currently, Kas uses Mailbox ("fast vsync") present mode; this does not however have a frame limiter (it can hit 3000-4000 FPS with a decently large window on my system).

The new kas-soft backend (#610) also needs a frame limiter of some sort (not supported by softbuffer: rust-windowing/softbuffer#299).