ColorAxis - Issues #69 & #70 - Canvas is null fix, and Allow flipping of color axis for canvas controls

Added a null check for canvas in the UpdateVisuals() method.  

Added a property FlipColorScheme to the ColorAxis class.  Apply's a layout rotation transform of 180* to the rectangle that contains the colors to be shown in the control.  Allows the colors in the color axis to be aligned with the applied material texture coordinates without having to create a separate color brush.