Merge FifoCommandRunnable into Decode

No idea if this change is desirable.
I still think it's this issue here. You fetch the remaining size in Decode itself:
> GetVideoBufferEndPtr() - g_pVideoData

This should be moved completely out of scope (maybe on Fifo.cpp?), so that both, Decode and OpcodeDecoder_Run, will get the size of this video buffer. Everything else can/should be calculated internally.