avcodec/avcodec: Store whether AVCodec->close needs to be called

avcodec/avcodec: Store whether AVCodec->close needs to be called

Right now all AVCodecContexts except those using frame-threaded decoding
call the codec's init function and expect its close function to be
called. In order to make sure that the close function is not called for
frame-threaded decoding ff_frame_thread_free() resets
AVCodecContext.codec (and because of this it has to free the private
AVOptions of the main AVCodecContext itself). This is not obvious and
potentially fragile. Instead add a field to AVCodecInternal that
indicates whether close should be called for this AVCodecContext.
It is always zero when using frame-threaded decoding, so that resetting
the codec is no longer necessary and has been removed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>