1) If the window is resized to a size which is not equal to the skin's size, opacity mask controls are shown stretched.
2) The ScrollContentPresenter uses ScissorRects to clip its contents. But ScissorRects cannot be combined with LayoutTransforms/RenderTransforms, so we have to use another technique here. We could use an opacity mask when drawing the contents.

h4. Steps to Reproduce
1) Open the MP2 client window and make it bigger in the horizontal dimension. All screen contents will be streched correctly except the mirror of the header label which will remain in the same size.
2) Same as 1), look at the main menu which gets incorrectly clipped at the right side