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.
video: fix some vo_direct3d crashes due to FFmpeg nonsense

For incomprehensible reasons, AV_PIX_FMT_GRAY8 (and some others) have a
palette. This literally makes no sense and this issue has bitten us
before, but it is how it is.

This also caused a crash with vo_direct3d: this mapped a texture as
IMGFMT_Y8 (i.e. AV_PIX_FMT_GRAY8), and when copying this, it tried to
copy the non-existent palette.

Fixes #1113.
video: fix redrawing when pausing after framedrop

When pausing after a frame was just dropped, we're logically at the
dropped frame, and thus should redraw the dropped frame. This was
implemented, but didn't work after unpausing for the second time,
because of a minor logic bug.
manpage: clarify vf_pp docs

Make the note about quoting more prominent.
Add a warning that this filter is useless for new files.
TOOLS: idet: remove extra '$' in $(()) expansion
options: remove --volstep

It's just confusing; users are encouraged to edit input.conf instead
(changing the argument to the "add" command).

Update input.conf to keep the old behavior.
command: always show OSD message when changing volume etc.

If there's a command that uses the OSD by default, then always print the
associated message (or a fallback made of name + value), even if the
command has an associated OSD bar.

This means volume, gamma, panscan, etc. all show both a message and a
OSD bar.

Also, add a '%' to the volume message. The extra_msg thing is not needed
anymore.

See issue #1103.
command: remove unneeded ifdefs

Why bother?

Also, since now some properties could be mapped to non-existing options,
but mp_property_generic_option() is used, deal with this case and return
a not-found error code.
command: allow changing some OSD-related options even with no VO

If no VO was open, these options couldn't be changed or even queried.
Although these properties are nearly useless if no VO exists, there's
actually no good reason to forbid querying or setting them. Also, even
if the VO is created, it doesn't mean the VO window was created.