Add ability to control camera manually (e.g. with keyboard)

[Plonq]

Some of this was already possible, but there were a couple issues:

• Constraints (upside down, and min/max alpha/beta) only applied to mouse input. Fixed by always applying these constraints.
• Modifying radius on its own didn't do anything. Fixed by adding a force_update option which forces recalculation of rotation and translation. This also technically applied to the focus, but since you only notice where the focus is when you rotate, it didn't make any difference.
• It wasn't possible to zoom orthographic cameras by setting radius, so I had to change it so the orthographic projection's scale is based off the radius.

Adds keyboard control example.