OpenGL2: Small glsl shader optimizations, fixes, and cleanup.

code/renderergl2/glsl/fogpass_fp.glsl

@@ -5,5 +5,5 @@ varying float var_Scale;
 void main()
 {
 	gl_FragColor = u_Color;
-	gl_FragColor.a *= sqrt(clamp(var_Scale, 0.0, 1.0));
+	gl_FragColor.a = sqrt(clamp(var_Scale, 0.0, 1.0));
 }