Don't overwrite nlogo files with png when using all-previews in 3d

Running `all-previews` in headless 3d is not very useful (at least until, if ever, #833 is completed), but this will at least avoid messing up the models themselves.

src/main/org/nlogo/headless/ChecksumsAndPreviews.scala

@@ -47,7 +47,7 @@ object ChecksumsAndPreviews {
       List("HUBNET", "/GOGO/", "/CODE EXAMPLES/SOUND/")
         .forall(!path.toUpperCase.containsSlice(_))
     def remake(path: String) {
-      val previewPath = path.replaceFirst("\\.nlogo$", ".png")
+      val previewPath = path.replaceFirst("\\.nlogo(3d)?$", ".png")
       val workspace = HeadlessWorkspace.newInstance
       try {
         // we set the random seed before opening the model, so that the random-seed will affect the