Fix issue #294: Canvas not working when not rendering to default render target.

src/modules/graphics/opengl/Canvas.h

@@ -49,6 +49,27 @@ namespace opengl
 		bool loadVolatile();
 		void unloadVolatile();
 
+		/// Unbind the current canvas on construction and rebind it upon destruction.
+		// Exception safe temporary unbinding/rebinding for techniques that require
+		// the default canvas to work (e.g. stencils).
+		// XXX: If a canvas is switched between creating and destroying an unbinder
+		//      object, the canvas will not be switched back.
+		struct TemporaryUnbinder
+		{
+			Canvas * canvas;
+			inline TemporaryUnbinder() : canvas(Canvas::current)
+			{
+				if (NULL != canvas)
+					Canvas::bindDefaultCanvas();
+			}
+
+			inline ~TemporaryUnbinder()
+			{
+				if (NULL != canvas && NULL == Canvas::current)
+					canvas->startGrab();
+			}
+		};
+
 	private:
 		friend class PixelEffect;
 		GLuint getTextureName() const { return img; }