Create the application scene through a renderer plugin

createApplicationScene takes a renderer plugin, defaulting to Cesium,
which builds today's viewer and credits exactly as before. Its renderer
is transitional: it has only the members it really has, with read-only
capabilities, and a test pins the contract members still to come. Code
not yet ported reaches the viewer through cesiumViewerOf, never through
a renderer member. A plugin whose renderer has no viewer is refused and
destroyed, since the application still runs on the viewer. The result
now includes the renderer.

Not yet checked in a browser.

scripts/package-boundaries.json

@@ -2421,6 +2421,8 @@
       "src/app/layers/traffic.js",
       "src/app/operations.js",
       "src/app/scene.js",
+      "src/app/renderer.js",
+      "src/render/contract/index.js",
       "src/sources/sourceSlot.js",
       "src/app/sources.js",
       "src/app/startupChrome.js",