PolyLine shifting can crash

[dabreegster]

make_polygons(0.5m) of PolyLine { pts: [Pt2D { x: 5217.6908, y: 5241.742 }, Pt2D { x: 5217.683, y: 5241.7357 }, Pt2D { x: 5212.1705, y: 5244.2894 }], length: Distance(6.0854) } failed: Line from Pt2D { x: 5217.6908, y: 5241.742 } to Pt2D { x: 5217.683, y: 5241.7357 } too small

Camden and other maps crash for reasons I don't understand -- the road center line is OK, but when we try to shift the polyline, the result doesn't break upfront, but it later crashes when trying to make polygons.

Short-term fix is to avoid the crash and just return a circle instead of a thickened polyline:

Actual fix is to make polyline shifting correct and robust. I never meant for my horrible math to last this long; I really want to investigate https://github.com/jbuckmccready/cavalier_contours or something else.

[dabreegster]

Another example where a bug somewhere in this implementation is causing a mess:

It's actually fine in the map_editor:

(So maybe this is the problem with trimming center points from both ends?)

It looks fine in OSM, https://www.openstreetmap.org/node/282416719#map=19/51.46319/-2.54542

[dabreegster]

trimmed_road_geometry in the map_editor is totally fine for this road, even after shifting the result. There's some difference happening elsewhere in the importer. I am baffled.

[dabreegster]

In map_editor, trimmed points are

PolyLine::new(vec![     // length 120.6564m
  Pt2D::new(2608.9265, 2146.7426),
  Pt2D::new(2726.1538, 2140.5341),    // 117.22730000000001, -6.2085000000001855 (+ 117.3916m @ Angle(356.96837717355965 degrees))
  Pt2D::new(2728.6358, 2138.9885),    // 2.481999999999971, -1.5455999999999221 (+ 2.9239m @ Angle(328.08850253534797 degrees))
  Pt2D::new(2728.7598, 2138.6709),    // 0.12399999999979627, -0.31759999999985666 (+ 0.3409m @ Angle(291.32707776309627 degrees))
])

that matches importing by the end of InitialMap. OK. Over in the final map... Road.center_pts also match. Great, so the problem is somehow in the code that calculates lanes...

[dabreegster]

Here we go! For the lanes, we first shift the center points to the left edge of the road, then gradually shift it right. That's causing the problem here -- the center points are OK, shifting them left is mostly OK but winds up with that bend. If we then shift the partly bent line, the bend explodes.

[dabreegster]

This is if we shift relative to the road center. The yellow center-line is still funky, because of get_left_side...