This resolves pixelation issues on some mobile devices (fixes #725).
It also may be related to issues #301, #496, #683, and #708.

For three event listeners added to the container elements weren't being
removed properly, since `removeEventListener` was mistakenly being called on
`uiContainer` instead. This was preventing garbage collection.

Add `stopMovement` to stop all movement

This allows for equirectangular images on a given device to be twice as big as
they were previously, so (almost) all devices now support equirectangular
images up to 8192px across.

This technique could be extended to double the maximum size again (using eight
textures), but images bigger than 8192px should be converted to the
multi-resolution format anyway, so there's little point in implementing this
extension.