Add ShumateVectorReader API

Adds an API to give applications low-level access to the contents of vector tiles.

Apps can create a ShumateVectorReader from a GBytes, then use iterate() to get a ShumateVectorReaderIter. The ReaderIter has methods to get layer information, loop through features, and get feature IDs and tags. There's also three methods to deal with geometry: get_feature_geometry_type(), feature_contains(), and get_feature_point(). Currently there is no public API to get the raw geometry as a series of coordinates/instructions.

I've also updated the vector renderer to use the new API. It uses private methods to access feature geometry.

Also fixed an otherwise unrelated bug that came up during testing.