Graphics: Fix viewport effect rendering bug

We didn't account for the spec dictating that
scissor test does affect FBO blit operations,
which resulted in corrupted output if more than
one viewport with an active effect (tone, color)
was created.

The reason I never caught this before must be
that the fglrx-legacy driver is actually bugged
in this aspect and ignores the scissor on blit.

src/graphics.cpp

@@ -175,7 +175,13 @@ public:
 	void requestViewportRender(Vec4 &c, Vec4 &f, Vec4 &t)
 	{
 		pp.swapRender();
+
+		/* Scissor test _does_ affect FBO blit operations,
+		 * and since we're inside the draw cycle, it will
+		 * be turned on, so turn it off temporarily */
+		glState.scissorTest.pushSet(false);
 		pp.blitFBOs();
+		glState.scissorTest.pop();
 
 		PlaneShader &shader = shState->planeShader();
 		shader.bind();