Libavfilter for ffplay support.

This still needs some minor work here and there but should be already functional.
Note that the code pathes that are under "not avfilter" ifdefs as well as the
ifdefs will be droped as soon as all major issues have been det with, aka could
be real soon or not.

Originally committed as revision 22216 to svn://svn.ffmpeg.org/ffmpeg/trunk