I am currently using the Path class to implement (alpha blended) selections on a GUI and it works OK otherwise but I can't figure out any nice way to make an inverse fill out of the path. (To allow for inverting the user selection.)
Not sure I understand - do you want your lassoed area to effectively cut out of the full rectangle? If so, then draw the rectangle, change the winding rule with setUsingNonZeroWinding() and then add the lasso path.
Starting from a state like shown in the screenshot, I would like to switch the filled in area to be outside the freeform selection, leaving the lassoed area in its original state.