This also adds an overlay element to show progress since this model can take a while to load.
Add shadows to the FlightHelmet demo, add a link to it on the site, and add recently-introduced entry points to the JS bindings.
This is a more complex test than suzanne because it has multiple materials and exercises the named materials functionality in loadFilamesh. This also exercises our glTF-to-Filamesh pipeline at build time. See issue #663