please test master

[Boudewijn Rempt]

Hi,

After a deep discussion last night I've pushed an experiment patch to 
master that should improve painting performance a bit (not a huge amount, 
but still useful). However, I really need to know whether it causes 
regressions.

So, if you're using git master, please update and report on issues when 
painting, especially with low opacity strokes and undo/redo or other 
brushes than the pixel brush. Issues can only appear when painting in wash 
mode, not build-up mode.

Thanks!

Boudewijn