marshal geojson ids into mvt uint64 if possible

[paulmach]

The encoding/mvt package converts geojson into mapbox vector tiles (mvt). The problem is geojson allows ids of strings and numbers while mvt only allows uint64 (positive numbers). This is a common incompatibility between the two types.

For marshaling, the change is to try to convert the geojson.Feature.ID into an uint64. If it's a number type and positive, it converts to unit64. If it's a string it tries to decode.

Cavets:

• If the number is negative the feature ID will be omitted.
• If the number is a decimal, e.g. 123.45. the encode id will be the truncated, e.g. 123.

Unmarshaling code was also updated to decode ids into float64 to be consistent with what json does with numbers. That way if you have the same data in geojson or mvt the decoded types will be the same.

@jerluc

[jerluc]

@paulmach this looks great, and I definitely appreciate the best-effort approach to encoding the ID properly depending on the original type.

Hopefully in MVT v3, this should conform better to the GeoJSON specification, but for now, this'll be a very helpful bridge.