Updating default density to 1 loop for 4m for improved performance

[TheDuckCow]

I find that applying a density of 4 (which reads "1 mesh loop every 4 meters") is a better default than our current 2x, which has twice as many loop cuts. At 4m with the default lane width also of 4m, it makes for nearly square but long rectangles.

at density=2 (previewws)

at density=4 (new)

Of course, really sharp turns will still look a little low poly on the outside edge. And turns with roads with a higher number of lanes will also tend to look courser. But I do feel this is still a good general default starting place. In wheel steal game, for instance, we have a density of 3 but if not for the discontinuity of CSG mesh creation, would push it higher.

[TheDuckCow]

With the second commit, I've also changed the default gutter profile to extend down only 0.5m instead of the prior 2.0m, which felt a bit cartoonish and at the very least would require more beveling to look ok in non lowpoly contexts. This does mean by default there isn't quite as much margin to work with the terrain, but that's really the responsibility of the terrain creator in the end.

Before:

After: