composite: Delay the triangle until a bit later

Firefox seems to fire the sendEvents out of band, so let's just increase
our hacky sleep() to make it work. At some point I should build this
actor properly.
composite: Disambiguate a use of the word "Composite"

I didn't notice this before when I was writing it, but the RENDER
extension has a completely unrelated protocol request called "Composite"
which lets us composite two pictures together, like, alpha blending and
Porter-Duff Compositing "Composite".

Add a bit more prose here to make the intended meaning clearer.