cellPngDec: Ignore bytesPerLine < width * bpp.

It seems strange, but Sonic CD passes bytesPerLine = w * 3 for a 4 channel
image.  Either this is a bug elsewhere, or it ignores likely incorrect
strides.

Untested except in Sonic CD.
Merge pull request #242 from unknownbrackets/pngdec

cellPngDec: Ignore bytesPerLine < width * bpp