Changes to CustomData:
Some functions would only return the current active layer, added extra variants that take an index to select the level (modeled after CustomData_get_layer_n.)
Still todo:
* UVs are being interpolated linearly, should probably offer Catmull-Clark subdivision like Subsurf modifier.
* Vertex Colors still don't support multiple customdata layers.
* Editing UV data on levels other than 1 should be disabled in the interface (same for weights)