Serious performance regression on Linux

[janisozaur]

There is a severe performance hit on Linux. It seems to be directly connected with the size of PaintArea - the larger the PA is, the slower the drawing is. Perhaps on update whole PA is being redrawn instead of only invalidated region. This is unlike windows build, where everything is fine regardless of PA size.
As an outcome of this, seemingly some mouse events are either lost or discarded, resulting in blocky painting (yet again, this only concerns mouse and not the tablet)