pthread: drop avcodec_ prefixes from static functions
lavc: split slice and frame threading functions into separate files
pthread_slice: rename ThreadContext -> SliceThreadContext

This should prevent confusion with frame threading.
pthread: store thread contexts in AVCodecInternal instead of AVCodecContext

It's a private field, it should not be visible to callers.

Deprecate AVCodecContext.thread_opaque
lavc: move AVCodecContext.pkt to AVCodecInternal

It's a private field, not meant to be accessed from outside lavc.
aacdec: Set the profile during decoding

Previously the profile would not be set if the bitstream needs
to be decoded to know the profile.

Signed-off-by: Anton Khirnov <anton@khirnov.net>