Fix First Person camera controls for Z-up
Fix First Person camera controls for Z-up

This fixes the camera controls of the First Person camera with up_axis
set to `Vector3::z()`. The method `set_up_axis` now also panics when a
value other than `Vector3::y()` and `Vector3::z()` is given as they are
not supported.

See also: Issue #122

sebcrozet/kiss3d