MARPLOT BINARY LAYER FILE FORMAT: SM2 (summary of summary) files

This file may not always exist for a given layer on a given map. When it does,
it is a list of bounding rectangles, one rectangle for each 1000 summary
records.  Each rectangle is the union of its 1000 constituent object rectangles.
The format of each rectangle is:

	low-long + 180000000 (long)
	low-lat + 90000000 (long)
	hi-long + 180000000 (long)
	hi-lat + 90000000 (long)

1000 * <number of SM2 records> may be less than <number of SUM records>, but
not greater.