vtdec: Register a hardware-only vtdec_hw on OSX and give it a higher rank

vtdec: Register a hardware-only vtdec_hw on OSX and give it a higher rank

while having the default vtdec at secondary rank. This allows decodebin/playbin
to prefer the hardware based decoders, and if that fails to initialize because
hardware resources are busy to fall back to e.g. the libav based h264 decoder
instead of the software based vtdec (which is slower), and only fall back to
the software based vtdec if there is no higher ranked decoder available.