Fixed MLAA in GLES renderer.

This function was used by Cand for his OpenGL 2.1 renderer. The round function is available since GLSL 1.3 (OpenGL 3.0), so there is no need to override it.
If it will be needed in future, I suggest to change the name to round_stk or something similar.

data/shaders/mlaa_blend2.frag

@@ -56,11 +56,6 @@ float SearchYUp(vec2 texcoord) {
 	return min(i - 1.5 + 2.0 * e, 2.0 * MAX_SEARCH_STEPS);
 }
 
-
-vec2 round(vec2 invec) {
-	return vec2(floor(abs(invec) + vec2(0.5)) * sign(invec));
-}
-
 vec2 Area(vec2 distance, float e1, float e2) {
 	// * By dividing by areaSize - 1.0 below we are implicitely offsetting to
 	//   always fall inside of a pixel