lavc/pngdec: always create a copy for APNG_DISPOSE_OP_BACKGROUND

lavc/pngdec: always create a copy for APNG_DISPOSE_OP_BACKGROUND

Calling av_frame_make_writable() from decoders is tricky, especially
when frame threading is used. It is much simpler and safer to just make
a private copy of the frame.
This is not expected to have a major performance impact, since
APNG_DISPOSE_OP_BACKGROUND is not used often and
av_frame_make_writable() would typically make a copy anyway.

Found-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>