KisAutoBrush: override maskWidth and characteristicSize too

This fixes the autobrush from getting clipped when there are more than 2 spikes.
Ideally, we'd move KisQImagePyramid::imageSize to KisBrush and make it virtual.
maskHeight, maskWidth, and characteristicSize would be implemented using that
(and not vritual any more) ensuring that they are mutually consistent.