Merge pull request #14633 from greggman/distance-fog

distance fog

src/renderers/shaders/ShaderChunk/fog_fragment.glsl

 #ifdef USE_FOG
 
+	float fogDepth = length(fogPosition);
+
 	#ifdef FOG_EXP2
 
 		float fogFactor = whiteCompliment( exp2( - fogDensity * fogDensity * fogDepth * fogDepth * LOG2 ) );

src/renderers/shaders/ShaderChunk/fog_pars_fragment.glsl

 #ifdef USE_FOG
 
 	uniform vec3 fogColor;
-	varying float fogDepth;
+	varying vec3 fogPosition;
 
 	#ifdef FOG_EXP2
 

src/renderers/shaders/ShaderChunk/fog_pars_vertex.glsl

 #ifdef USE_FOG
 
-  varying float fogDepth;
+  varying vec3 fogPosition;
 
 #endif

src/renderers/shaders/ShaderChunk/fog_vertex.glsl

 
 #ifdef USE_FOG
-fogDepth = -mvPosition.z;
+fogPosition = mvPosition.xyz;
 #endif
\ No newline at end of file