Simplify buildings algorithm

[systemed]

This adds a specialised simplification algorithm for buildings, which aims to preserve their rectilinear nature while reducing vertices. You can try it out using a layer config like this:

"building": { "minzoom": 13, "maxzoom": 14, "combine_polygons_below": 15,
              "simplify_algorithm": "buildings", "simplify_below": 15, "simplify_level": 0.0001 }

It works best with combine_polygons_below. Typical output is like this:

from this source data:

The algorithm was developed in Ruby, and ported to C++ with the help of Claude.

Performance is moderate - there are some opportunities for optimisation (such as replacing atan2 with a faster implementation), but given that the main impact is from combine_polygons_below anyway, this isn't a massive deal.

[systemed]

Additional commit now optimises union_many by using an rtree to only join adjoining polygons.

[systemed]

Some very rough timings based on running Oxfordshire on an old Mac mini:

no combine, no simplify:  15.19 real   36.76 user   1.89 sys   22085632
no combine, simplify:     22.28 real   38.77 user   3.13 sys   22216704
old combine, no simplify: 65.96 real  129.22 user   3.95 sys   21233664
old combine, simplify:    67.17 real  127.34 user   4.15 sys   20840448
new combine, no simplify: 18.38 real   44.76 user   1.67 sys   21233664
new combine, simplify:    19.97 real   43.63 user   2.35 sys   20709376