Remove MEncoder

Remove MEncoder

Disable MEncoder compilation and remove files used by MEncoder
only. There's no attempt to remove all references to MEncoder from the
build system, documentation etc at this point.

Removed files: (muxers, audio/video encoders, misc)

mencoder.c
cfg-mencoder.h
parser-mecmd.[ch]
xvid_vbr.[ch]
libmpdemux/muxer*
libmpcodecs/ae*
libmpcodecs/ve*
libmpcodecs/native/rtjpegn.[ch]
libmpcodecs/native/mmx.h   // was used by rtjpegn only

Rationale:

MEncoder is still useful for some people, but there's not much
potential for further development; in the long run almost all use
cases can be handled better by solutions based on something else (for
example using FFmpeg or encoding MPlayer output). FFmpeg is already
getting video filtering support which should work for some common
MEncoder uses.

Keeping MEncoder working takes extra work that is away from player
development. While that amount of work is not huge (mostly MEncoder
can be just ignored), it's not completely insignificant either.

MEncoder is still maintained to some degree in the svn tree, so if
necessary it's possible to use it from there for now. This tree has
never had major improvements for the MEncoder side, so using svn
MEncoder instead should be no major loss.