Partially fix mod_libavcodec: it can now render files without crashing. However there...

Partially fix mod_libavcodec: it can now render files without crashing. However there be gremlins in that thar code, for example if initialisation fails, synfig will return an error message as it should and then crash on program shutdown! Also, encoding audio with the video is not supported at all, since the code that does that is disabled with comment and #ifdefs.

git-svn-id: http://svn.voria.com/code@287 1f10aa63-cdf2-0310-b900-c93c546f37ac