Share the stub's pointer routing as an engine-neutral input router

The stub's lease and press routing moves to src/render/input/router.js,
unchanged, so a real renderer can feed it its own engine's events. The
router also keeps the behaviour holds (telling the renderer when one turns
off or on, unless it lacks it) and ctrlWheelZoom, and dispatch says who an
event went to.

scripts/package-boundaries.json

@@ -5178,6 +5178,7 @@
       "src/render/stub/geodesic.js",
       "src/render/stub/viewRectangle.js",
       "src/render/stub/orbits.js",
+      "src/render/input/router.js",
       "src/render/conformance/goldens/camera.json",
       "src/render/conformance/checks/camera.js"
     ],