Sky material: Don't render sky twice; cull back faces. Also disable useless depth writing.

jme3-core/src/main/resources/Common/MatDefs/Misc/Sky.j3md

@@ -9,10 +9,6 @@ MaterialDef Sky Plane {
         VertexShader GLSL100:   Common/MatDefs/Misc/Sky.vert
         FragmentShader GLSL100: Common/MatDefs/Misc/Sky.frag
 
-        RenderState {
-            FaceCull Off
-        }
-
         WorldParameters {
             ViewMatrix
             ProjectionMatrix
@@ -23,7 +19,10 @@ MaterialDef Sky Plane {
             SPHERE_MAP : SphereMap
 	    EQUIRECT_MAP : EquirectMap
         }
+
+        RenderState {
+            DepthWrite Off
+            DepthFunc Equal
         }
-    Technique {
     }
 }