When extracting images from QIS, one may get rectangles that are not part of the
GDSII/OASIS file. These rectangles are due to filtering features of QIS and can be 
avoided with the proper QIS settings.

There are 2 ways to not draw the "filtering rectangles":

1. To set the display filter size to 0.  This way, all detail polygons will be drawn
   (instead of just one rectangle representing the whole area).  The command to control
   this is Set_Display_Filter_Size (and Set_Structure_Display_Filter_Size for cell
   references).

   In our viewer, we have a "Detail Mode" control to turn off all display filters and
   draw all details thinking that users would zoom in to an area and only click
   "Detail Mode" to view all details when necessary.

   Setting display filter to 0, or a lower value say 2 or 4, would slow down the drawing
   speed because more polygons are drawn.

2. To leave filter settings at a reasonable value (5, 10 or 20) but don't draw the
   filtering rectangles.

   The effect of this is that areas filtered out would now be empty.  Detail polygons in
   these areas are not drawn and the whole area is left blank.

   Use the command Set_Draw_Filtered_Areas to control this.  Arguments can be On, Off.
   On is to draw filtered areas as rectangles.  Off is to leave all filtered areas blank.
   Default is On.