Fix #435 (for real this time). Unfixes #324.

Add a extern vec2 love_ScreenSize to the pixel effects that will
be (re)set upon PixelEffect::loadVolatile() and is used to flip
the fragment coordinate.

Unfixes #324 because canvases can have different dimensions than
the screen.

src/modules/graphics/opengl/Quad.cpp

@@ -113,17 +113,6 @@ void Quad::flip(bool x, bool y)
 	}
 }
 
-void Quad::mirror(bool x, bool y)
-{
-	for (size_t i = 0; i < NUM_VERTICES; ++i)
-	{
-		if (x)
-			vertices[i].s = 1.0f - vertices[i].s;
-		if (y)
-			vertices[i].t = 1.0f - vertices[i].t;
-	}
-}
-
 const vertex *Quad::getVertices() const
 {
 	return vertices;