Rotate/zoom around selected element

[jdumas]

Oftentimes when I'm debugging/inspecting something real close, it becomes hard to navigate the camera, since any rotation centered on the scene and any rotation will quickly drift the view away from the element I'm trying to inspect. It would be good if we could, e.g., select an element (vertex, face, etc.) and have the camera rotate/zoom around it instead, which would facilitate inspection. I don't know if Polyscope has such a feature yet, but I think it'll be super useful.