Interpolate Direct Mode drawer closing

This is pretty simple to interpolate, since ed.dmtileeditor is
guaranteed to have only changed by 1, and then it's just multiplied by
4.

desktop_version/src/editor.cpp

@@ -2961,7 +2961,7 @@ void editorrender(void)
         {
             if(ed.dmtileeditor<=4)
             {
-                t2=(4-ed.dmtileeditor)*12;
+                t2=graphics.lerp((4-ed.dmtileeditor+1)*12, (4-ed.dmtileeditor)*12);
             }
 
             //Draw five lines of the editor