player: fix cover art sometimes "syncing" against audio

Cover art is treated like video, but is not really video. In one case,
the audio sync code was accidentally still active. Fixes cover art
playback with --ao=null. (This is due to ao_null's latency emulation.
Although it's not very clear whether that is actually correct...)
old-configure: we accidentally used bash features

In at least 2 places, we want to "shift" positional parameters by a
certain amount, or remove all parameters if there are fewer parameters.
bash handled this just fine, but dash vomits upon it. POSIX shell allows
both behaviors, so we have to fix it.

Fix a comparison done with "==" instead of "=".
vo_vdpau: always allocate the black pixel

black_pixel is an (apparently necessary) 1x1 black surface used for
clearing the screen. It was allocated in RGB mode only, but is sometimes
used in YUV mode too.
vdpau: move RGB surface management out of the VO

Integrate it with the existing surface allocator in vdpau.c. The changes
are a bit violent, because the vdpau API is so non-orthogonal: compared
to video surfaces, output surfaces use a different ID type, different
format types, and different API functions.

Also, introduce IMGFMT_VDPAU_OUTPUT for VdpOutputSurfaces wrapped in
mp_image, rather than hacking it. This is a bit cleaner.
timer: fix (usually impossible) timespec.tv_sec overflow

This usually can't happen, because even if time_us (first input value)
is INT64_MAX, the value added to tv_sec will be about 2^43, and tv_sec
will be <2^31, far below a possible overflow in 64 bits. But should
time_t be 32 bits (32 bit Linux/Windows?), an overflow could happen.
tl_mpv_edl: correct misspelling on line 233

Closes #783

Signed-off-by: wm4 <wm4@nowhere>
player: give quit_watch_later an exit code argument like quit

The quit command has an optional argument that is used as exit code.
Extend that to the quit_watch_later command. Actually, unify the
implementations of the two commands.

Requested in #798.