Format of a Group3 file produced by pstog3
==========================================

file		::=	header page*

header		::=	"!<Group3>" vsn xdpi ydpi
vsn		::=	"1"
xdpi		::=	"200"
ydpi		::=	"100" | "200"

page		::=	line line ... line rtc fill	    (1170 lines for low res, 2340 for high res)
line		::=	EOL data			    (1728 dots-worth of encoded data)
rtc		::=	EOL EOL EOL EOL EOL EOL		    (six EOLs)
fill		::=	"0"*                                (pad to byte boundary)