Mostly ported from mpv, but some parts were slightly rewritten. Notably,
the way window resizing is handled. Instead of having two different
sources of window size information and having to argue about which one
to take as authoritative, we only use the current extent as reported by
the vulkan implementation.

Apparently, not all platforms report valid values here - particularly,
wayland apparently doesn't. If somebody has an idea for how to figure
out the size of a wayland window, feel free to tell me; because the
vulkan wayland extension doesn't seem to consider it being possible.