Encapsulate code from WebMDemuxer to query keyframe and frame resolution inside VPXDecoder, so we have a clean wrapper for all the libvpx functions we use.
This creates a number of stubs and leaves some surrounding code that may be irrelevant (eg. recorded time stamps, status variables).
Stub resolution/removal should be a follow-up to this.
According to FFmpeg documentation, the out parameter is "set to size of parsed buffer or zero if not yet finished." however this is only the case if no error occurred; otherwise it is left untouched. We want the invalid content to generate a decoding error, so we set size to inputSize to ensure decoding failed later.
