Speed up image selection pulls from the document by changing the transparency mask cache to be the actual mask used for the transparent image - rather than a "brush" used to set mask pixels to transparent.

I have no idea why I originally opted for the more complex "brush" method when this is so much easier and executes faster.

Basically, we've inverted the meaning of each pixel of the cache, and transparent pixels of the base image are now transparent in this cache, rather than being treated like any other document pixel.

This means we don't have to use the deprecated and slow kpPixmapFX::paintMaskTransparentWithBrush() anymore.