Remove useless video filters

Most of these have very limited actual use, or are even entirely
useless. They only serve to bloat the codebase and to make life harder.
Drowning users in tons of barely useful filters isn't exactly helpful
either. Some of these filters were redundant or marked as obsolete.

The dlopen and lua (to be added soon) video filters provide ways to add
custom filters.

Detailed listing for each filter with reasons (with contributions from
divVerent and lachs0r):

1bpp:
    Replaced by "scale".
2xsai:
    Pixel art scaling algorithm, useless with lossy video.
blackframe:
    Not very useful. Apparently one use is combining it with scripts,
    that pass the
bmovl:
    Weirdly complex and insane (using FIFO commands), questionable use.
cropdetect:
    Only sort-of useful when used with scripts, and then it will be
    very fragile.
    It's probably better to use the dlopen rectangle filter, or to
    implement the common use-case in a better way.
decimate:
    Not needed/useful with modern video codecs, is an
    encoding-only filter.
denoise3d:
    "hqdn3d" is better.
detc:
    Some of the worse deteleciners.
dint:
    Useless, actually crashes. (On an assert in vf.c that is disabled
    by default in mplayer-svn.)
dvbscale:
    Not even practical, and the same effect can be achieved through
    other means.
eq:
    Worse/older version of eq2.
field:
    Limited use, available as dlopen filter.
fil:
    Quoting the manpage:
        This filter is very similar to the il filter but much faster,
        the main disadvantage is that it does not always work.
        Especially if combined with other filters it may produce
        randomly messed up images, so be happy if it works but do not
        complain if it does not for your combination of filters.
filmdint:
    Kind of redundant with pullup, and slightly worse.
fixpts:
    Never useful. (Most if not all filters have been fixed for PTS.)
framestep:
    Questionable use. For things like creating thumbnails, ffmpeg or
    --sstep should be used.
geq:
    Limited use, will be redundant with the "lua" filter.
halfpack:
    Useless, probably redundant with "scale".
harddup:
    Useless.
hue:
    Most VOs support this.
il:
    Useless.
ivtc:
    Another of the worse deteleciners.
kerndeint:
    A bad deinterlacer.
lavc:
    For DVB output devices. We removed that support.
lavcdeint:
    A bad deinterlacer, was already deprecated.
    Still available as --vf=pp=fd.
mcdeint:
    A broken deinterlacer that uses lavc internals.
ow:
    Very slow, barely any quality benefit over "hqdn3d".
palette:
    Done by "scale".
perspective:
    Files with incorrect perspective are extremely rare. About the
    only real-world use for this is keystone correction, which is
    usually done in hardware by the projector or by graphics
    drivers/compositors.
pp7:
    Another useless postprocessing filter with bad and complicated code.
    Use libpostprocess with "pp" instead.
qp:
    Useless.
remove-logo:
    Redundant with delogo, which is better and more practical.
rgbtest:
    Useless.
sab, smartblur, boxblur:
    Blur filters, redundant to "unsharp".
softskip:
    Does nothing.
spp, fspp, uspp:
    Useless postprocessing filters. "spp" needs ffmpeg internals.
    "fspp" is the optimized version of the "spp" filter (???), while
    "uspp" is the slow version (????).
    Use libpostprocess with "pp" instead.
telecine:
    Evil and useless. Available as dlopen filter for testing
    purposes.
test:
    Useless.
tfields:
    Useless, probably.
tile:
    Questionable use. Available as dlopen filter.
tinterlace:
    Evil and useless.
yuvcsp:
    Probably useless.
yvu9:
    Redundant with "scale".

Also remove the following left-over files: vd_null.c, vqf.h
mplayer: fix crash when muted and audio codec fails

When audio codec initialization fails, reinit_audio_chain() will call
uninit_player() to close the AO. mpctx->ao is set, but mpctx->mixer.ao
is still NULL. uninit_player() assumes both variables are always the
same, and calls mixer_uninit(), even though mpctx->mixer.ao is NULL.
That function tries to access the ao without NULL check if mute was
enabled.

Fix this in mplayer.c by not relying on the assumption that
mpctx->ao == mpctx->mixer.ao. Also, add a check for NULL to mixer.c
(function muxer_uninit()). One of the checks is redundant and only one
of them is needed, but we add both for general robustness.