Fix aquarium in Firefox. Only enable multiview when VR is supported

[brianpeiris]

The Aquarium sample is currently broken in Firefox on Ubuntu using Nvidia drivers with the following error.

Error compiling shader:
0(3) : error C0210: extension GL_OVR_multiview2 not supported for version 150
[log.js:69:22](https://webglsamples.org/tdl/log.js)

Apparently Firefox thinks OVR_multiview2 is available to use on Ubuntu since gl.getExtension('OVR_multiview2') returns with an extension object.

This PR changes the sample code to use multiview only if VR support is detected. Tested with Firefox and Chrome on Ubuntu, Windows, Vive Browser on Vive XR Elite, and Cardboard. Would appreciate help testing on a Quest device.

[kenrussell]

Thanks for this fix Brian. It looks good to me but Brandon, could you double-check?

[toji]

LGTM as well, thanks! It's unfortunate that Firefox is reporting OVR_multiview2 as supported when it's not.

[brianpeiris]

Thanks for the merge!