Timeline: Always resize tracks Fl_Pack.

This eliminates a bug were all tracks are invisible if the resized
window results in the tracks pack being shorter than its original
height.

Timeline/Timeline.C

@@ -886,6 +886,8 @@ Timeline::resize ( int X, int Y, int W, int H )
     /* why is THIS necessary? */
     hscroll->resize( BX, BY + H - 18, hscroll->w(), 18 );
     vscroll->size( vscroll->w(), H - 18 );
+
+    tracks->resize( BX, rulers->h(), W - vscroll->w(), H - vscroll->h() );
 }
 
 /** draw ancillary cursors (not necessarily in the overlay plane) */