Added a channel mask feature to GafferImageUI::ImageView.

This commit allows the user to view different channels by pressing the corresponding key for R,G,B and A. It is implemented by modifying the outColor variable within the fragment shader of GafferUI::StandardStyle. 
This addresses issue #403.

 * Added an optional parameter to GafferUI::Style::renderImage() that allows a colorMask to be passed in. The default value enables all of the channels.
 * Added a pretty little icon on the image viewer's info bar to indicate which channel is being viewed.
 * Fixed a bug that caused the text that displays the mouse's position to be cleared when the viewer updates.