app_server: allow replacing the DrawState in a Canvas

* Needed for layers support. The previously set DrawState and its
  stack predecessors are not freed, so take care to not leak memory
  when using this.

src/servers/app/Canvas.cpp

@@ -88,6 +88,13 @@ Canvas::PopState()
 }
 
 
+void
+Canvas::SetDrawState(DrawState* newState)
+{
+	fDrawState = newState;
+}
+
+
 void
 Canvas::SetDrawingOrigin(BPoint origin)
 {

src/servers/app/Canvas.h

@@ -41,6 +41,7 @@ public:
 	virtual	void			PushState();
 	virtual	void			PopState();
 			DrawState*		CurrentState() const { return fDrawState; }
+			void			SetDrawState(DrawState* newState);
 
 			void			SetDrawingOrigin(BPoint origin);
 			BPoint			DrawingOrigin() const;