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.cpp

@@ -407,6 +407,7 @@ void editorclass::reset()
     script.customscripts.clear();
 
     returneditoralpha = 0;
+    oldreturneditoralpha = 0;
 
     ghosts.clear();
     currentghosts = 0;
@@ -4636,6 +4637,7 @@ void editorinput()
                         music.haltdasmusik();
                         graphics.backgrounddrawn=false;
                         ed.returneditoralpha = 1000; // Let's start it higher than 255 since it gets clamped
+                        ed.oldreturneditoralpha = 1000;
                         script.startgamemode(21);
                     }
                 }