Make icons as UI not change worldspace size when zooming far out.

Icons as UI looks good around the usual unit-to-icon transition, but it makes the icons too large when zoomed out. The issue is that the icons are drawn at a static size in screenspace, which causes their worldspace size to increase as the camera is zoomed out. The old icons were drawn in worldspace so do not have this problem.

To get the best of both systems I would like a setting that causes icons to maintain their worldspace size beyond a configurable zoom level. I expect this would take a bit of smoothing and fiddling to look good as a sudden threshold that switches to shrinking the icons in screenspace may look jarring. Ideally the icons would be able to gradually shrink in screen space before doing the worldspace size maintenance behaviour.

Start by tracing the icon rendering path and the camera zoom handling described in the issue, comparing the current screen-space behavior with the former world-space behavior. Done means icons smoothly transition toward maintaining their world-space size beyond a configurable zoom level without a jarring threshold.

Written by the indexing model from the issue text.