Remove love_ScreenSize from shaders.

PixelEffects would fail to load when screen_coords were unused.

Reopens issue #435: "PixelEffects have flipped screen_coords.y".

src/modules/graphics/opengl/PixelEffect.cpp

@@ -136,10 +136,6 @@ bool PixelEffect::loadVolatile()
 
 	glDeleteShader(shader);
 
-	Graphics *instance = (Graphics*)Module::getInstance("love.graphics.opengl");
-	GLfloat screenSize[2] = {(float)instance->getWidth(), (float)instance->getHeight()};
-	sendFloat("love_ScreenSize", 2, screenSize, 1);
-
 	return true;
 }