Babylonjs demos update

[paulrouget]

This PR adds a new copy of the babylonjs demos.

The new copy includes 3 new demos, removes the skull demo, updates the bbjs library, and also the playground code.

I want to keep the old babylonjs demos around for some time, it will be useful when we run into regressions, to know if it's a servo regression, or a demo regression.

To access the old demos, use https://servo.org/hl-demos/index.old.html

fix https://github.com/servo/servo.org/issues/91

[paulrouget]

I've checked with the emulator, and all the demos load. But the new demos are not really usable yet as they require interaction.

[jdm]

Thanks!