[x3d-public] Shadows

[Andreas Plesch]

As motivation to make Shadows a high priority for the next version let
me share Holger's cool idea to use Sponza for shadows:

https://create3000.github.io/media/examples/Navigation/NavigationInfo/example.html

I could adopt it pretty much as is for x3dom:

https://raw.githack.com/andreasplesch/x3dom/scopedLights/test/functional/scopedLights/inline.html

(without the nice ParticleSystem, and with slightly changed
intensities and locations).

The main required field for both x3dom and x_ite to enable shadows is
shadowIntensity. 0 means no shadows, 1 means full shadow.

-Andreas
-- 
Andreas Plesch
Waltham, MA 02453