Sorting rectangle-hilbert pairs in TPIE rtree

[mrevs]

The functions create_rstartree and create_hilbert_rtree in bulkloader.h both sort a sequence of rectangle-hilbert pairs using TPIE sort. I assume that the intention was to order the pairs in order of hilbert values. However since no comparison object is given to TPIE sort, it would seem that the pairs are ordered according to the < operator of pairs (and thereby the < operator of rectangles since pairs are lexicographically ordered).

I guess it is easy to fix by either changing rectangle-hilbert pairs to hilbert-rectangle pairs or providing a comparison object to TPIE sort.