prep/finalizeImageData: add support for caller overriding automatic premultiplication handling

If alpha handling is unnecessary, functions can gain some speed by
overriding automatic handling of premultiplication reversal.  However,
any function that changes image alpha (e.g. Blur, etc) should never use
this, or invalid alpha values will result!

This feature is primarily for effects and adjustments that only affect
RGB values.
pdSelection: reduce debug messaging
Pixelate effect: fix to process the alpha channel, if one exists

Without this, the resulting effect is uuugly on 32bpp images.