avcodec/x86/pngdsp: fix off by 1 error

This fixes artifacts in the last pixel of rows with some widths and pixel formats

Found-by: Dominique Leroux <Dominique.Leroux@autodesk.com>
Tested-by: Dominique Leroux <Dominique.Leroux@autodesk.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: handle 4 more bytes with SIMD for RGB64 in paeth prediction

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