Graphics: Fix transition when new scene has gray tone effect

Rendering the scene may cause PingPong swaps of front / backbuffer,
so don't take references to those buffers until after rendering.

Fixes #199

src/graphics.cpp

@@ -708,16 +708,16 @@ void Graphics::transition(int duration,
 
 	setBrightness(255);
 
+	/* Capture new scene */
+	p->screen.composite();
+
 	/* The PP frontbuffer will hold the current scene after the
 	 * composition step. Since the backbuffer is unused during
 	 * the transition, we can reuse it as the target buffer for
 	 * the final rendered image. */
 	TEXFBO &currentScene = p->screen.getPP().frontBuffer();
 	TEXFBO &transBuffer  = p->screen.getPP().backBuffer();
 
-	/* Capture new scene */
-	p->screen.composite();
-
 	/* If no transition bitmap is provided,
 	 * we can use a simplified shader */
 	TransShader &transShader = shState->shaders().trans;