flatgeobuf as a source format

One of the performance bottlenecks of using Tippecanoe is generating the GeoJSON input, which is both expensive (performance wise) to produce and I assume for tippecanoe to read (though I must say, it's still very fast).

I commonly use ogr2ogr to read a range of input formats (frequently ESRI FileGDB) and pipe out GeoJSONSeq for tippecanoe to read, however this is slow.

The performance of [flatgeobuf](https://github.com/bjornharrtell/flatgeobuf) looks really compelling, if Tippecanoe could naively read it, that might prove to improve performance greatly for some use cases.

Start with Tippecanoe's input path and the FlatGeobuf documentation linked in the issue, comparing them with the existing ogr2ogr-to-GeoJSONSeq workflow. Done means Tippecanoe can read FlatGeobuf natively and the performance benefit for relevant use cases is demonstrated.

Written by the indexing model from the issue text.