This fixes the emscripten binding errors that we've been seeing
with the <filament-viewer> test page, which prevented us from
including web in the last few Filament releases.
The binding errors were caused by double-initializing the emscripten
module.
I fixed this by allowing clients (e.g. FilamentViewer) to call
Filament.init() more than once. We now accumulate a list of "on ready"
callbacks that get triggered after the emscripten module becomes ready.
As far as I can tell, multiple canvases were actually always broken, and
the viewer test page worked in the past only because we got lucky.
327576274f