Support libav, too

av_frame_alloc() exists in ffmpeg, and exists on the libav master branch
too, but does not yet exist in any released version of libav.

Test whether the symbol exists; if it doesn't, simulate it using calloc.

HACK: the actual av_frame_alloc() uses refcounting to release the data,
but we can't easily simulate that; so define an av_frame_dealloc that
does nothing in the "we have an av_frame_alloc" case, but releases the
data again in the other case, and call that from appropriate
destructors. This is slightly ugly, but it should work.
Run the test suite when building the package
Remove commented-out code

That's why SCM exists