OpenGL2: Shader optimization, and add dither to tonemap.

code/renderergl2/glsl/calclevels4x_fp.glsl

@@ -15,7 +15,7 @@ vec3 GetValues(vec2 offset, vec3 current)
 #ifdef FIRST_PASS
 
   #if defined(USE_PBR)
-	minAvgMax = pow(minAvgMax, vec3(2.2));
+	minAvgMax *= minAvgMax;
   #endif
 
 	float lumi = max(dot(LUMINANCE_VECTOR, minAvgMax), 0.000001);