vid_vsync: fully support adaptive, remove polling, print better errors

vid_vsync: fully support adaptive, remove polling, print better errors

Adaptive vsync was partially supported: it worked if set before window
creation but not if toggled on afterwards, and wasn't mentioned in the
cvar description.

The immediate application of `vid_vsync` cvar changes was implented by
polling, which was ok for one cvar but I'll be making more vid_ cvars
apply immediately (no vid_restart needed), and don't want to poll for
things that rarely change every frame.

SDL is now queried for the current state of vsync instead of storing
what we believe to be the current state.

If the setting can't be applied the reason (as reported by SDL) is now
printed.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>