Get a measurement for testing whether two points can snap together, useful for choosing the best snap point among multiple options. For perspective mode, the returned metric is the "visual angle" between the points, which is the angle between the points to the camera scaled such that the visual angle between the horizontal bounds of the view is 90. For orthographic mode, it is a projected distance onto the view plane scaled such that the distance between the horizontal bounds of the view is 90. Thus, the metric is suitable for comparing against a visual angle snap threshold to determine if snapping should happen.