Point handles for routes and outlines; Area corners; bus goes round right

Studio: one selected object with pts (a route, an outlined area) shows a
handle on each point and a + in the middle of each side: drag a point to
move it, drag a + to add one there, double-click a point to remove it
(open routes keep 2, outlines 3). Points snap to the grid and onto wall,
fence and road ends, at any angle; one undo step each. Handles keep their
screen size (new viewer callback onView on every redraw) and follow
re-renders and selection. The viewer captures the pointer, so a double-
click targets the drawing: the last point pressed is remembered instead.

Prefab meta `outline`: the prefab draws pts as its outline. Area has it:
Edit corners turns its rectangle into four points, Rectangle again
removes them.

Routes: where a path doubles back, the lane offset puts a point on each
side, so it rounds into a U-turn instead of a diagonal across the lanes.
The streets demo bus now runs east, three quarters round the roundabout
(clockwise, left-hand traffic), down south street, turns and comes back
by the short way, instead of going round the wrong way on its way out.

Tests: Rust 81 (a doubling-back route: each mover once, no sort cycles),
Studio 134 (corners, move, add, remove, undo), web 100.

examples/streets/scene.json

@@ -33,7 +33,7 @@
     {"id": "lamp-4", "type": "lamp", "x": -44, "y": 5.2},
     {"id": "lamp-5", "type": "lamp", "x": 5.2, "y": -28},
     {"id": "traffic", "type": "route", "name": "Traffic round the block", "x": 17.0, "y": 23.0, "pts": [[3.0, -23.0], [17.0, -23.0], [17.0, 23.0], [-5.0, 23.0], [-11.0, 21.392], [-15.392, 17.0], [-17.0, 11.0], [-17.0, -3.0], [-12.408, -11.913], [-5.913, -18.408]], "props": {"kind": "car", "n": 4, "closed": true, "path": false}},
-    {"id": "bus-line", "type": "route", "name": "Bus line", "x": -25.0, "y": -20.0, "pts": [[-25.0, 20.0], [5.0, 20.0], [13.913, 15.408], [20.408, 8.913], [25.0, 0.0], [25.0, -20.0]], "props": {"kind": "bus", "n": 1, "path": false, "colour": "car4"}},
+    {"id": "bus-line", "type": "route", "name": "Bus line", "x": -19.456, "y": -14.457, "pts": [[-30.544, 14.457], [-0.544, 14.457], [8.369, 19.049], [14.864, 25.544], [24.048, 25.544], [30.543, 19.049], [30.543, 9.865], [24.048, 3.37], [19.456, -5.543], [19.456, -25.543], [19.456, -5.543], [14.864, 3.37], [8.369, 9.865], [-0.544, 14.457]], "props": {"kind": "bus", "n": 1, "path": false, "colour": "car4", "closed": true}},
     {"id": "motos", "type": "route", "name": "Motorbikes", "x": 35.75, "y": -11.75, "pts": [[-8.25, 8.25], [8.25, -8.25]], "props": {"kind": "motorbike", "n": 3, "path": false}},
     {"id": "cyclists", "type": "route", "name": "Cyclists", "x": 46.5, "y": 46.0, "pts": [[-9.5, 0.0], [9.5, 0.0]], "props": {"kind": "cyclist", "n": 2, "path": false}},
     {"id": "strollers", "type": "route", "name": "People strolling", "x": 38.0, "y": 23.0, "pts": [[0.0, -17.0], [0.0, 17.0]], "props": {"kind": "person", "n": 3, "path": false}}