Combine edge pair in FGB export

[ianthetechie]

We should really only need one geometry. I initially wanted to try exporting both edges to make it easier but this will OOM on a planet scale due to how FGB indexes are currently built (RAM needed initially scales with number of features).

To combine the edges we will need to re introduce the opposite edge fetch, write access based on the forward / reverse perspective consistently, and write two edge ID fields.

[michaelkirk]

In case it's interesting, see flatgeobuf/flatgeobuf#317 for discussion on writer memory consumption.

I've also been slowly gardening the geomedea format, which is largely inspired by my experiences with flatgeobuf. It includes a memmapped backed index writer. Presumably something like this could be added to FGB as well.

[ianthetechie]

Yeah, I found that issue on the FGB memory usage after I noticed a problem 😅

[ianthetechie]

And very cool new format 🤔 I completely agree on the compression choice FWIW; I like most things about FGB but it's a bit annoying that it's not compressed internally in any way. Not wrong per se but annoying for large scale data.

Will keep an eye on that. Though honestly our only reason for using FGB here at the moment is as a fast way to make the Valhalla graph legible to other software like QGIS or Tippecanoe.