Aquarium: Support for multiview VR rendering

This implements multiview rendering using WebGL 2.0 and the
WEBGL_multiview extension. Antialiasing is disabled since multiview
doesn't support that yet.

Supporting VRUI in multiview mode is left for later.

Multiview needs to be enabled by including the query parameter
enableMultiview=true in the Aquarium URL.

Author: Olli Etuaho