For anyone interested in an open source mapping library (that also has 3D support) that can handle raster data (including the Mapbox tiles), I launched a project 2 weeks ago that is the culmination of 7 years effort: https://github.com/felixpalmer/procedural-gl-js/.
I must say (while acknowledging my huge bias) that I'm disappointed with how the 3D terrain works in the new Mapbox version. It is the top-billed feature, but the controls feel clunky, the terrain often disappears and they didn't even bother implementing distance fog to give the scene scale. When they announced the 3D feature last year I was expecting more.