Fixes our post processing shaders so they work under OpenGL ES 3.0

Most of our shaders relied on implicit int->float conversions.
This fixes the assumption that this is allowed

Data/Sys/Shaders/chrismas.glsl

@@ -10,7 +10,7 @@ void main()
 	float green = 0.0;
 
 	if (c0.r < 0.35 || c0.b > 0.35)
-		green = c0.g + (c0.b / 2);
+		green = c0.g + (c0.b / 2.0);
 	else
 		red = c0.r + 0.4;
 	ocol0 = vec4(red, green, 0.0, 1.0);