Destroy everything

Following up https://github.com/tangrams/tangram/commit/20f577127304e5ecffd47b0b2f32c06487b2a1df & https://github.com/tangrams/tangram/commit/f22d80e16319686d100d09578f3e22f0aed482cc with more scene, GL resource, and leaflet layer clean-up.

It's now safe to remove the leaflet layer (however, the layer currently cannot be re-added to the map; we may want to add support for that, which will require re-initializing the scene). It's also safe to individually delete different scene components (a texture, render mode, program, etc.) without causing GL or JS errors.
This is required to put leaflet in a "ready" state - without this, it will never call the layer's ```onAdd()``` method. Maybe I should put a comment in the source too.