clean up RageDisplay methods, add ability to mask song banner

The masking uses the Z-buffer under the assumption that some cards don't have a stencil.  Should we be using the stencil instead?  It doesn't really matter because we never need both the Z buffer and stencil buffer simultaneously.

stepmania/src/CourseContentsList.cpp

@@ -96,7 +96,7 @@ void CourseContentsList::Update( float fDeltaTime )
 void CourseContentsList::DrawPrimitives()
 {
 	// turn on Z buffer to clip items
-	DISPLAY->EnableZBuffer();
+	DISPLAY->SetZBuffer( true );
 
 	// write to z buffer so that top and bottom are clipped
 	m_quad.SetZ( 1 );
@@ -128,7 +128,7 @@ void CourseContentsList::DrawPrimitives()
 	}
 
 	// turn off Z buffer
-	DISPLAY->DisableZBuffer();
+	DISPLAY->SetZBuffer( false );
 }
 
 void CourseContentsList::TweenInAfterChangedCourse()