Various game engine fixes:

* Fix issue with add transparency mode with blender materials.
* Possible fix at frontface flip in the game engine.
* Fix color buffering clearing for multiple viewports, it used
  to clear as if there was one.
* Fix for zoom level in user defined viewports, it was based on
  the full window before, now it is based on the viewport itself.
* For user defined viewports, always use Expose instead of
  Letterbox with bars, the latter doesn't make sense then.

source/gameengine/Rasterizer/RAS_IRasterizer.h

@@ -168,6 +168,10 @@ public:
 	 * BeginFrame is called at the start of each frame.
 	 */
 	virtual bool	BeginFrame(int drawingmode, double time)=0;
+	/**
+	 * ClearColorBuffer clears the color buffer.
+	 */
+	virtual void	ClearColorBuffer()=0;
 	/**
 	 * ClearDepthBuffer clears the depth buffer.
 	 */
@@ -181,7 +185,8 @@ public:
 	 */
 	virtual void	EndFrame()=0;
 	/**
-	 * SetRenderArea sets the render area from the 2d canvas
+	 * SetRenderArea sets the render area from the 2d canvas.
+	 * Returns true if only of subset of the canvas is used.
 	 */
 	virtual void	SetRenderArea()=0;
 
@@ -195,6 +200,7 @@ public:
 	 * @return true if stereo mode is enabled.
 	 */
 	virtual bool	Stereo()=0;
+	virtual bool	InterlacedStereo()=0;
 	/**
 	 * Sets which eye buffer subsequent primitives will be rendered to.
 	 */