CHG: Refactor ffmpeg MT support

/cc @fritsch 

This adresses 2 issues:
- On Android (and probably ios/osx, @Memphiz ?), the h/w decoders are in specific codecs, so we only get into ffmpeg as a fallback, with sw decoders only. This addresses the fact that it was impossible to specify mt sw decoding for fallback
- The recently added m_videoDisableSWMultithreading is fundamentally a duplicate of videoplayer.useframemtdec. As videoplayer.useframemtdec is specifically for s/w decoding, use this one and allow modification even if H/W decoding is selected, for the fallback case.