avformat/img2_alias_pix: Add AVClass and image2 AVOptions

This fixes the default timebase and allows overriding the framerate

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: print the actual number of bytes left in the error case

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/png_parser: Return the buffered data at the end

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