Compatibility with "classic GIS file formats"

I think we need to think about compatibility with "classic GIS file formats" like Shapefiles. What I mean by that is all those formats that only support one type of geometry per layer and maybe even only one layer per file. It should be well defined how those files map to Geobuf files. I am **not** saying we should limit ourselves to what those files support. But I think it would be useful to define a subset of the Geobuf format that is guaranteed to map well to those formats. Maybe even define some flag that can be set promising that the file contents behave in that way.

Start by reviewing the Geobuf format and the compatibility requirements described in this issue, including Shapefiles and other single-geometry or single-layer formats. The scope is to agree on how that subset maps to Geobuf and whether a flag is needed; no files or tests are identified.

Written by the indexing model from the issue text.