mplayer: fix looping of very short files

Commit 884c179 attempted to make it possible to skip backwards through
the playlist, even for files which fail to intitialize, or play for a
very short time. This was also used to prevent mpv from looping forever
and doing nothing if --loop=inf is used, and no file in the playlist is
playable.

This broke looping of very short files, because mpv was assuming that
this case happened. But there are legitimate use cases.

Fix this by making the looping case special. Instead of checking whether
playback was "very short", check whether something could be decoded and
displayed/played. If yes, allow looping.
Remove completely outdated AUTHORS file

Considering that we use version control, maintaining an AUTHORS file
doesn't make much sense, and indeed it was barely touched since the time
mplayer2 forked from MPlayer. None of the contributors to mplayer2 or
mpv are listed in this file (even uau's work is not appropriately
reflected, despite having an entry in this file).

I'm removing this file because it's obviously pointless to keep around,
and moreover it's misleading.

In theory removing this file might be questionable: the git log reaches
back into CVS and SVN times, which don't have separate committer and
author fields for each comment. The commit messages could be incomplete
and not mention the original author, while the AUTHORS file somehow
could. I doubt this is a real concern, since you can always reconstruct
the original AUTHORS file from git.

Another concern is that this could be interpreted as disrespectful to
the people mentioned in this file; but it isn't meant to be
disrespectful.

Also point out a slight irregularity in the Copyright file.
vf_scale: factor out libswscale equalizer control

Will be used by vo_x11.
x11: remove colormap code, always request TrueColor visuals

vo_x11 had a clever trick to implement a video equalizer: it requested a
DirectColor visual. This is a X11 mechanism which allows you to specify
a lookup table for each color channel. Effectively, this is a safe
override for the graphic card's gamma ramp. If X thinks the window
deserves priority over other windows in the system, X would temporarily
switch the gamma ramp so that DirectColor visuals can be displayed as
the application intends. (I'm not sure what the exact policy is, but in
practice, this meant the equalizer worked when the mouse button was
inside the window.)

But all in all, this is just lots of useless code for a feature that is
rarely ever useful. Remove it and use the libswscale equalizer instead.
(This comes without a cost, since vo_x11 already uses libswscale.)

One worry was that using DirectColor could have made it work better in
8-bit paletted mode. But this is not the case: there's no difference,
and in both cases, the video looks equally bad.