The function is setting the brushes correctly in the return value, but
without updating the resolve_mask, making it return wrong results in
functions like isBrushSet or the debug operator.
Added a unit test for the member function, since the class is still
mostly untested, and clarified the reference documentation of what the
function is supposed to do.
Change-Id: Iaa820dc44f095e125f9375cb00da5569986803c6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
7f34e8b58c