avmux: Don't fail on commas in the libav muxer type names

https://bugzilla.gnome.org/show_bug.cgi?id=697512
libav: Use new flush vfunc of video codec base classes and remove reset implementations
tests: Enable avdemux_ape test again, it works now

https://bugzilla.gnome.org/show_bug.cgi?id=684684