remove gstffmpegdecall. This was a temporary hack to get around some issues with our video/avi-centered mimetype syst...

Original commit message from CVS:
* remove gstffmpegdecall. This was a temporary hack to get around
some issues with our video/avi-centered mimetype system. Now that
we use proper identifiers, we don't need that piece of cruft anymore.
* update ffmpeg codec map for ffdec_*, plus make the uncompressed stream
caps also be formed by the codec mapper (this is easier to handle imo).
* add audio support (untested, though :P).
* remove the copying of buffer content and add a get_buffer() handler
instead.
* add an int to ffmpegenc to make it compile and resolve the symbol. I'll
truely fix that later on.
* make mimetypes in the ffmpeg codec mapper conform to docs/random/mimetypes
* etc.

Oh, and let's not forget that it can play back DIVX5 et all nicely. Support
for SVQ1/3 is also there, so if someone completes qtdemux, we can watch
these, too.