Cherry-picked commits from markembling/photosphere (Part 2)

- Keeping a partial photosphere in frame (min/max longitudes) markembling/photosphere@303105e1422c73aaa7ef9f7f3c9e2ef65f9056c8
- Images, fix for north-facing camera markembling/photosphere@7fdb55d75266fab993740c0ee86db766155be34d
Suitable (although not perfect) demo images.
Fix for not correctly calculating max north latitude (and thus not
displaying images' tops when requested)

- More work at initial placement and tidying markembling/photosphere@63c73cd19655913a5aab50c6a4c323f2988cec95
- Proper start point calculation finally markembling/photosphere@c2360a93701a0cfffabd991214841ff4e4699a79

Note: Only changes to sphere.js and the additional sample images have been picked.