libavcodec/pngdec: support 'previous' dispose operation for APNG.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: fix mem leak in init()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: fix indentation in handle_row()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>