avcodec/pngdec: use memcpy instead of byte loops for P frames.

Rely on the way memcpy is optimized for one's system instead of looping
on a byte buffer for buffer copies to handle P frames.

Tested-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>