R3F Components: Consider adding a screen-space ruler component

[gkjohnson]

For giving a sense of scale a la Google Maps:

Note that this may make sense only when the camera is oriented in in a top-down or orthographic view where the rule scale correctly applies to the entirety of the rendered image. Otherwise the ruler distance will only apply to a single position like the center of the camera view.

This discrepancy would ideally be clearly conveyed to the user. In fact it's not clear in the google maps view what the scale applies to when the camera is tilted (with a perspective camera). It could also update as the user moves their mouse to reflect the scale of the items under the mouse and be hidden otherwise if we don't know what the scale applies to.

[gkjohnson]

Terriajs implements this here and chooses the two sibling pixels at the bottom center of the screen to determine lat / lon and measure distance between the two points using a geodesic. I'm not convinced using the chord length is the best approach for reflecting distances, though.

This strategy makes sense at close distances but when zoomed out far from the globe it may make most sense to shift to using a center point.