Backporting of features to C API / Consolidating go-mapnik packages

[fawick]

Hi @olt, I just noticed your package and I really like the features you have added.

This is not really an issue with omniscale/go-mapnik, but have you considered offering your modifications to @springmeyer 's mapnik-c-api back upstream? I believe that the C-API really would benefit from the things you haved added, as would all derivative works.

[springmeyer]

I just added both of you as collaborators at https://github.com/springmeyer/mapnik-c-api. Would be great if you could create pull requests for any changes you see that would be improvements. I'll review and you can merge :)

[olt]

We needed a Mapnik integration in Go with a few more options (layer selection, render options etc.) and both the C-API and your go-mapnik lacked important features for that. So, the easiest way was to just implement the API in a single repo and change what we need.

The current API is now stable and we use it in production. I'd love to merge features back.

I'm not sure how useful the layer selection feature would be, because it won't work as expected with Mapnik 3, since deactivated layers are not available from the Mapnik API anymore.