Reorganize the code such that the frame threading code does not call the
decoders directly, but instead calls back into the generic decoding
code. This avoids duplicating the logic that wraps the decoder
invocation and allows receive_frame()-based decoders to use frame
threading.
Further work by Timo Rothenpieler <timo@rothenpieler.org>.
thread.h currently contains both API for decoder use and functions
internal to lavc generic layer. Move the latter to avcodec_internal.h,
which is a more appropriate place for them.
The goal is to distinguish between APIs provided by the generic layer to
individual codecs and APIs internal to the generic layer.
Start by moving ff_{decode,encode}_receive_frame() and
ff_{decode,encode}_preinit() into this new header, as those functions
are called from generic code and should not be visible to individual
codecs.
