Restore Lighting.j3md fog in SinglePass by fixing SPLighting varying linkage (#2771)

* Initial plan

* Fix fog varying name in SPLighting shader

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Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

Author: Copilot

jme3-core/src/main/resources/Common/MatDefs/Light/SPLighting.vert

@@ -10,7 +10,7 @@
 
 // fog - jayfella
 #ifdef USE_FOG
-varying float fog_distance;
+varying float fogDistance;
 uniform vec3 g_CameraPosition;
 #endif
 
@@ -201,6 +201,6 @@ void main(){
     #endif
 
     #ifdef USE_FOG
-    fog_distance = distance(g_CameraPosition, (TransformWorld(modelSpacePos)).xyz);
+    fogDistance = distance(g_CameraPosition, (TransformWorld(modelSpacePos)).xyz);
     #endif
 }