Consider supporting OpacityMask with MultiEffect

The mask in MultiEffect matches to ThresholdMask (https://doc.qt.io/qt-6/qml-qt5compat-graphicaleffects-thresholdmask.html) except that it has additional min & max threshold & spread and also invert property. Idea was that it would work also as simpler opacity mask by setting "maskThresholdMin: 0.5; maskSpreadAtMin: 1.0". But with closer look this doesn't match exactly to OpacityMask output.

As OpacityMask is quite often used, consider extending MultiEffect API with "maskMode" property taking in Threshold/Opacity enums. Threshold is the default for compatibility. Then threshold & spread properties documentation should contain note "This property affects only when the maskMode is set to Threshold". Invert property should work for both masks.