GeometryTransformer that uses projections other than WSG84

[edoaxyz]

Hi, I'm trying to write MVT tiles with coordinates without any specific projection. I found that there's a class (TileGeometryTransform) that always tries to convert coordinates from WSG84 to local coordinates, and this behavior can't be changed because the instance of TileGeometryTransform is created on Write method. Also TileGeometryTransform is a struct (also internal!) so it cannot be even derived. Why not using NetTopologySuite.Geometries.Utilities.GeometryTransformer with all the Web Mercator stuff passed as an optional argument to Write and then compute all the differences between position and cursor on MapboxTileWriter ? IsPointInExtent and IsGreaterThanOnePixelOfTile can be put on writer class since they're used only there

[FObermaier]

According to v2 of the mapbox vector tile specification your request seems valid.

Could you work out a PR?