Replace FF_IDCT_LIBMPEG2MMX with FF_IDCT_AUTO

The former no longer exists.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Switch to refcounted frames and DR via get_buffer2

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Shutup GCC 4.8 type mismatch warning

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>