Smoothly interpolate "[Press ENTER to return to editor]" fadeout

Now it'll be real smooth at 60 FPS. Or above. Or whichever one you want
above 30.

desktop_version/src/editor.h

@@ -237,6 +237,7 @@ class editorclass{
   int dmtileeditor;
 
   int returneditoralpha;
+  int oldreturneditoralpha;
 
   std::vector<GhostInfo> ghosts;
   int currentghosts;