Make distance fog calculation overflow less in mediump

src/renderers/shaders/ShaderChunk/fog_fragment.glsl

 #ifdef USE_FOG
 
-	float fogDepth = length( vFogPosition );
+	// divide then multiply by max component to try to avoid saturating mediump precision
+	vec3 fogPositionAbs = abs( vFogPosition );
+	float fogMaxComponent = max( fogPositionAbs.x, max( fogPositionAbs.y, fogPositionAbs.z ) );
+	float fogDepth = length( vFogPosition / fogMaxComponent ) * fogMaxComponent;
 
 	#ifdef FOG_EXP2