Rename "wmav3" to "wmapro" in xine-lib's internals to line up xine-lib's
nomenclature with what everyone else calls it and knows it as.
[Tweaked by ds to avoid API change.]

Tell xine-lib that when it finds wmapro, look to ffmpeg.

ffmpeg's wmapro decoder is unique in that it puts out samples that
are floats, not 16-bit ints. These need to be converted.

This requires external ffmpeg.

There are two tricks to make VC1 decoding work:
1) VC1 sequence and entry point headers must be present in context->extradata.
2) video width and height must be known when opening decoder.

Some container formats store required extra data, but mpeg-ts does not.

1) is fixed by scanning the stream for headers and discarding all data until
proper headers are found.

2) is fixed by re-opening decoder with width and height information from
first open.