Fixed the subpixel precision bug when the canvas is mirrored

The problem was related to the fact that the size of the dab
changes depending on the value of the subpixel offset. This happens
due to various rounding complications. It means that if we want to keep
the hot spot of the dab unmoved, when the dab becomes mirrored, we should
change of x,y offset of the dab.

That is why the code that calculates the origin of the dab is now
moved to the KisDabCache, which handles all these complications.

BUG:323245