Introduced support for controlling mipmapping of osgTerrain::ImageLayer and compression of osgTerrain::HeightFieldLayer.

include/osg/GraphicsContext

@@ -267,7 +267,8 @@ class OSG_EXPORT GraphicsContext : public Object
         inline const Vec4& getClearColor() const { return _clearColor; }
         
         /** Set the clear mask used in glClear(..).
-          * Defaults to GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT. */
+          * Defaults to 0 - so no clear is done by default by the GraphicsContext, instead the Camera's attached the GraphicsContext will do the clear. 
+          * GraphicsContext::setClearMask() is useful for when the Camera's Viewports don't conver the whole context, so the context will fill in the gaps. */
         inline void setClearMask(GLbitfield mask) { _clearMask = mask; }
 
         /** Get the clear mask.*/