vo_vdpau: fix confusion around vsync_interval

vc->vsync_interval and vsync_interval should be the same value, but
actually vc->vsync_interval was updated after vsync_interval was
initialized. This was probably not intended. Fix this by removing the
duplicate local variable. There were probably no bad effects.
vo_vdpau: better integration with the generic framedrop code

vo_vdpau uses its own framedrop code, mostly for historic reasons. It
has some tricky heuristics, of which I'm not sure how they work, or if
they have any effect at all, but in any case, I want to keep this code
for now. One day it might get fully ported to the vo.c framedrop code,
or just removed.

But improve its interaction with the user-visible framedrop controls.
Make --framedrop actually enable and disable the vo_vdpau framedrop
code, and increment the number of dropped frames correctly.

The code path for other VOs should be equivalent. The vo_vdpau behavior
should, except for the improvements mentioned above, be mostly
equivalent as well. One minor change is that frames "shown" during
preemption are always count as dropped.

Remove the statement from the manpage that vo_vdpau is the default; this
hasn't been the case for a while.
TOOLS/umpv: use MPV environment variable, not UMPV_OPTIONS

Just like the rest of TOOLS/*.sh scripts.
TOOLS/umpv: print error message to stderr
TOOLS/umpv: make URL detection consistent with mpv method

See mp_is_url in options/path.c.