Build mkgmap/mkgmap #1271

Improve interpretation of TRE7 data (extended types offsets) not written by mkgmap - check magic value in TRE7 to find out which offsets are set (mkgmap write 0x607 which means all offsets are valid). GPXSee does something similar in TREFile::readExtEntry() and Steve already assumed this according to the comment for magic 0x0607 in TreHeader. - handle the case that the number of records in TRE7 is smaller than the number of subdivisions. In this case the subdivisions with the lowest resolution are probably without extended types. Should have no effect on mkgmap output but allows RgnDisplay to handle more maps.