vo_opengl_cb: free context on exit

Minor memory leak.

Actually, this also exposes some problems in the QtQuick example. This
will have to be fixed in the example.
client API: fix compiler warning if openglcb API is disabled

The function is void, and of course you can't return anything from it.
(Why does C or gcc even allow this?)
vo_opengl, vo_opengl_cb: check GL version in renderer

vo_opengl actually checks this in the context creation code already, but
it still increases robustness in case the requirements are changed
later.
vo_opengl_cb: parse renderer parameters

Now it accepts the same renderer arguments as vo_opengl. This also
disables debug checks by default, and reverts the background color
override. Both can now be controlled by the host application.
demux_mkv: support embedded coverart

The code could as well be in demux.c, but it's better to avoid
accidental clashes with demux_lavf.c.

FFmpeg provides no way yet to map a mime type to a codec, so do it
manually. (It _can_ map a mime type to an "input format", but not a
codec.)

Fixes #1374.
demux_mf: add probing by mime type

Reuses the mime type table introduced in the previous commit.