sd_ass: free external subtitle tracks
video: silence warnings for missing libavutils pixel formats

Using vf_screenshot on Libav printed useless/misleading error messages
when playing 10 bit h264 with a VO that supports 8 bit yuv420p only:

    Unsupported format 444p14le
    Unsupported format 444p14be
    ...

The cause of this is that vf_scale is inserted to handle the format
conversion, and tries to find a pixel format with best quality. This
includes the 14 bit and 12 bit formats, which don't exist on Libav.
vf_screenshot tries to query whether Libav's libswscale supports it,
resulting in these error messages. (In theory, vf_scale is missing this
check, but it doesn't matter in practice.)

Since this warning is rather useless anyway, because all input video
comes from libavcodec, and only the conversion into the other could
possibly fail. Silence the warning by raising it to verbose message
level.

Closes #7.
ass_mp: allow to use a custom fontconfig configuration file

This allows to use a fontconfig fonts.conf that is customized for mpv. The
configuration file is assumed to be located at `~/.mpv/fonts.conf`. If not
found the default fcontconfig config file is used.
core: path: run uncrustify

Even if this is not so bad as other files, I need to add some stuff so...
why not!?

`uncrustify -l C -c TOOLS/uncrustify.cfg --no-backup --replace core/path.h`
`uncrustify -l C -c TOOLS/uncrustify.cfg --no-backup --replace core/path.c`

The header was unchanged by the tool.
path: add mp_find_config_file and reorganize some of the code

Add `mp_find_config_file` to search different known paths and use that in
ass_mp to look for the fontconfig configuration file.

Some incidental changes spawned by this feature where:

 * Buffer allocation for the strings containing the paths is now performed
   with talloc. All of the allocations are done on a NULL context, but it still
   improves readability of the code.
 * Move the OSX function for lookup inside of a bundle: this code path was
   currently not used by the bundle generated with `make osxbundle`. The plan
   is to use it again in a future commit to get a fontconfig config file.
osxbundle: add fonts.conf to distributed bundle

The osxbundle target creates a bundle that is supposed to be distributable
to third parties. As they may not have fontconfig installed they miss a
fonts.conf pointing to the usual fonts directories in OSX.

For people installing from source and using from the terminal this commit
changes nothing. You just have to make sure that your fontconfig is installed
with a sane configuration (XQuartz does). If you are installing fontconfig from
source you can force a sane OSX default using `--with-add-fonts`. For example:

  `./configure --with-add-fonts=/Library/Fonts,~/Library/Fonts`

Homebrew already addressed this with mxcl/homebrew@b242883
osxbundle: print meaningful error when a dylib is missing

If one of the bundled libraries is pointing to a missing dylib stop the
bundling and exit with an error. This can happen if the user uninstalled a
dependency after he built the binary/libraries.
Merge remote-tracking branch 'origin/master' into sdl2
Merge branch 'sdl2' of ssh://github.com/mpv-player/mpv into sdl2