If an ImageBrush (or VisualBrush) is used for an OpacityMask, the texture coordinates are not matched correctly.

If the mask matches the render texture (full screen), the result is correct.

h4. Steps to Reproduce
See GUITestPlugin


h4. Additional Information
Default for ImageBrush is "Stretch=Fill".


When changing to "Stretch=None", the output is different, but Texture is scaled/cropped wrong:

      <Rectangle Grid.Column="0" Grid.Row="1" Fill="Red" Width="595" Height="240" Margin="10">
        <Rectangle.OpacityMask>
          <ImageBrush ImageSource="homebutton_mask.png" Stretch="None"/>
        </Rectangle.OpacityMask>
      </Rectangle>