Refined camera controls

- no stutter when click-dragging horizontally
- zoom in/out does not tilt camera up/down
- clicking and dragging to manually adjust camera view, releasing starts
up rotation from that point
- scales camera orbit semimajor axis by zoom level
- can switch between bodies by setting "cameraTarget = planet.planet;" or "cameraTarget = moonmesh;", etc. Anything with a ".position" property is valid for cameraTarget.