cellPngDec: fix outputMode, outputBytesPerLine

This handles CELL_PNGDEC_BOTTOM_TO_TOP and also handles reduced outputBytesPerLine more correctly-looking.  Flips textures correctly according to the [texture format test] (https://github.com/DHrpcs3/rpcs3/commit/d745a7bb4ebd974523506655de5b2071cbf47932#commitcomment-6359180).

Not actually tested against actual behavior for the outputBytesPerLine change, but I would be quite surprised if it was not handled this way.

-[Unknown]