sdl_viewer: Enable terrain rendering

[nnmm]

• Build terrain renderer directly into the tool, instead of making it an extension. This will allow us to remove the hacky extension system altogether.
• A terrain renderer has multiple layers that are drawn in sequence. The first one determines the transform – not sure if that is elegant, maybe we should just always initialize the camera above the point cloud bbox center?
• We need to know where the camera is in order to center the terrain window below it, hence the need for get_camera_to_world().

[nnmm]

ptal @krzesi-mir Last commit of this project! :)