Allow a <model> to specify a preference for spatial display

[zachernuk]

Given that some platforms must display content monoscopically (in such a way that could be composited into a page with CSS filters etc) and other platforms may display content stereoscopically/spatially, it would be useful for page authors to specify that preference.

In the event that it's useful to display a 3D model, but they'd also like to use CSS filters, use a transparent background or control aspects like a camera field of view, it would be unfortunate for a spatial platform to lose out on those capabilities. An attribute or other flag specifying a spatial projection could support this.

/facetoface