avcodec_set_dimensions() is supposed to be an internal utility function,
there is no reason whatsoever for it to be public. Therefore deprecate
it.

This can be optionally disabled whith the "output_corrupt" flags
option.  When in "output_corrupt" mode, incomplete frames are
signalled through AVFrame.flags FRAME_FLAG_INCOMPLETE_FRAME.
Signed-off-by: Anton Khirnov <anton@khirnov.net>