Spraycan Tool fixes:

* Fix the spray being too thick (the probability was being passed as the wrong argument to kpPainter::interpolatePoints())

* Remove the prevent-drawing-on-the-same-dot code in drawLineWithProbability(), on the grounds that:
a) no user will notice
b) it's a maintenance nightmare (the code was correct though)

* Speed up drawLineWithProbability() in the NOP case

* Reduce the spray thickness even further (now 5% instead of 10%)

* Style