Clip geometry and opacity mask support to declarative items.

It would be nice to have possibility to set clip geometry and opacity mask to declarative items.
Same way the Microsoft's WPF framework does it in UIElement class:

UIElement: http://msdn.microsoft.com/en-us/library/system.windows.uielement.aspx
Clip: http://msdn.microsoft.com/en-us/library/system.windows.uielement.clip.aspx
Opacity Mask: http://msdn.microsoft.com/en-us/library/system.windows.uielement.opacitymask.aspx

Current Qt Quick documentation says:
"Non-rectangular clipping regions are not supported for performance reasons."

It would be good to have these features supported in QML. If the performance related things are well documented the developer can easily make the decision whether to use the feature or not.