Fixed KisExperimentPaintOp to be much faster

Implemented an idea which we discussed long-long ago. The updates are
coming now only for the areas that really changed. In this tool the
changed area can be easily represented as a set of triangles. These
triangles are then split into a set of rectangles and the path is
rendered for these areas only.

Notes:
+ now the tool can be used by artists for real work
+ smoothing and speed options work perfectly
- mirroring doesn't work right now (will be fixed in the next patch)
- displacement doesn't work (I've disabled it, because efficient
implementation of it may take some time). I'll do it only if
artists will really need it.