Fixed a pressure bug in Wash-mode of painting

When the pressure drops, the opacity drops as well. In this moment, to
avoid the artifacts while painting the ALPHA_DARKEN op should work in a
reversed way. That is the contents of the layer should be painted on the
top of the brush dab. To allow this, we now keep the average value of the
opacity of several last dabs to be able to reverse the composite op.

This is tested to work best with spacings 0.1-0.2. With other spacing
values, probably, one needs to correct exponent value in
ParameterInfo::updateOpacityAndAverage().