Win: Colorpicker rainbow slider provides wrong coordinates

To reproduce just run the app and Jemmy Browser will open. After opening the custom color dialog just cilck refresh button in Jemmy Browser and then you can navigate through colorpicker.

As you may see from the video the rainbow slider provides wrong screenbounds where the red square is drawn. In doesn't coincide with its real place on the screene.

This bug is critical for colorpicker testing because colors are taken in particular points of the screen and then compared. Thus if we can't locate the real position of a slider knob then we will take the wrong color.