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/Quad.h

@@ -50,11 +50,6 @@ public:
 
 	virtual void flip(bool x, bool y) = 0;
 
-	/**
-	 * Mirror texture coordinates around 0.5
-	 */
-	virtual void mirror(bool x, bool y) = 0;
-
 	/**
 	 * Gets a pointer to the vertices.
 	 **/