Add transform option to toWKB and toWKT.

[flippmoke]

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Add the ability to provide a reprojection prior to using the toWKB and toWKT operations in src/mapnik_geometry.cpp similar to how you can provide the transform option for toJSON. Additionally add async functionality for both of these methods.

[springmeyer]

Yes, and good call on +async too.

[flippmoke]

Additionally it seems that toWKB does not provide any projection information, this is something that PostGIS expects apparently? Currently the workaround is to insert WKB objects as SRID=0 and then repoject after. Look into how we can provide either an SRID passed as an option, or have it known from the vector tile?

[artemp]

@flippmoke - I think not mixing re-projection and I/O would be better and more flexible solution - they are separate concepts. Having said that, I see some performance advantages re-projecting individual coords while iterating over them. How about providing `filter' adapter interface on our geometries where we can inject user defined functor to do re-projection or any other transformations ? Similar to STL approach

/cc @springmeyer

[springmeyer]

:-1: on adding SRID info the the WKB. We should not support EWKB - only supporting WKB output I think is fine. If programs want WKB (like PostGIS) then adding it via ST_SetSRID on the geometries provided by Mapnik should be fine.

:+1: to supporting fast reprojection. As far as how to do it I see two options:

1. Destructive/copy API

var geom = feat.geometry();
geom.transform(from,to);
geom.toWKB();

2. Filter API

Sketching out what I think @artemp has in mind:

var geom = feat.geometry();
var geom_filter = mapnik.GeomFilter({'type':'proj_transform','from':wgs84_proj,'to':merc_proj});
geom_filter(geom).toWKB();
geom_filter(geom).toGeoJSON();

Note: we'll need a solution here for both WKB and the existing feat.toJSON to keep existing functionality when we move on https://github.com/mapnik/node-mapnik/issues/421.