Sun offset tracking

[Carifio24]

This PR updates the tracking behavior when not centered on the Sun. Currently, we just keep the camera in place. This PR updates the behavior so that the "offset" of the current position from the Sun (offset here meaning the screen-space distance) is kept constant. The offset amount is updated each time we get a pointer up event on the canvas.

[patudom]

• If user changes location, WWT stops trying to keep your position relative to the sun steady. We should watch for that.
• If the Sun is offset from center, when user changes time with the slider, it's still pretty jittery (but is ok when playing time). See if this can be improved? - moved to new issue