vello_editor: Use PresentMode::Mailbox when available.

[xorgy]

AutoVsync/FIFO introduces unnecessary latency, especially on X11. There aren't animations in vello_editor so the downside (extra work) is not significant.

[tomcur]

As a data point: with this PR the example now works on my platform (Wayland on AMD GPU), previously it would nearly instantly crash with failed to get surface texture: Timeout, unless I changed AutoVsync to AutoNoVsync (and maybe Mailbox would've also worked).

[xorgy]

@tomcur that is certainly odd! AutoVsync should fall back to FIFO, which should be supported on all platforms including Wayland... but with that message it's apparently less about compatibility, but rather about missing a deadline.
As for AutoNoVsync, it's possible it selected Mailbox on your system.

[dfrg]

This LGTM. Fixes intolerable latency on Windows as well.