Remove London from screenshot testing, and use Cambridge instead. #449

This will make it easier to visually track the progress improving the
import. Originally London was added to have one left-hand driving map
under the test, but Cambridge works for that too, and it also includes
separate cycleways.

Also fix a crash when trying to draw very very tiny arrows.

data/MANIFEST.json

@@ -55,6 +55,11 @@
       "uncompressed_size_bytes": 14840548,
       "compressed_size_bytes": 3387178
     },
+    "data/input/cambridge/screenshots/trumpington.zip": {
+      "checksum": "a42f59cf1aab838872f7246baa17845c",
+      "uncompressed_size_bytes": 39274168,
+      "compressed_size_bytes": 39250810
+    },
     "data/input/krakow/osm/center.osm": {
       "checksum": "e64103d37a7009d96f7d8a653db02deb",
       "uncompressed_size_bytes": 56598855,
@@ -155,11 +160,6 @@
       "uncompressed_size_bytes": 3349383,
       "compressed_size_bytes": 786484
     },
-    "data/input/london/screenshots/southbank.zip": {
-      "checksum": "54bbcb9fd4b2eb8bceb474abb1724a03",
-      "uncompressed_size_bytes": 7148105,
-      "compressed_size_bytes": 7147592
-    },
     "data/input/nyc/osm/lower_manhattan.osm": {
       "checksum": "09bf8cdb40474741b58e74f4c486a69e",
       "uncompressed_size_bytes": 34156510,

game/src/debug/mod.rs

@@ -235,7 +235,7 @@ impl State<App> for DebugMode {
                             MapName::new("krakow", "center"),
                             MapName::seattle("lakeslice"),
                             MapName::seattle("montlake"),
-                            MapName::new("london", "southbank"),
+                            MapName::new("cambridge", "trumpington"),
                             MapName::seattle("udistrict"),
                         ],
                     ));

geom/src/polyline.rs

@@ -525,10 +525,9 @@ impl PolyLine {
         let head_size = thickness * 2.0;
         let triangle_height = head_size / 2.0_f64.sqrt();
 
-        if self.length() < triangle_height + EPSILON_DIST {
-            return None;
-        }
-        let slice = self.exact_slice(Distance::ZERO, self.length() - triangle_height);
+        let slice = self
+            .maybe_exact_slice(Distance::ZERO, self.length() - triangle_height)
+            .ok()?;
 
         let angle = slice.last_pt().angle_to(self.last_pt());
         let corner1 = self