Auto merge of #23169 - Manishearth:more-pose, r=jdm

Some more pose fixes

Based on https://github.com/servo/servo/pull/23164

This:

 - Adds support for position-disabled
 - Adds approximate support for floor-level
 - Makes transform.inverse.inverse return the original transform
(https://github.com/immersive-web/webxr/issues/576)

To support floor-level *properly* we have to decompose the `sitting_to_standing_transform` matrix. I'll have to add decomposition to euclid to do this, sadly.

It may be possible to reuse the decomposition code in servo's style crate, but there's a chance that that's written with column vector style, given that it reflects the CSS transforms spec, which is also written in column vector style. Ugh.

r? @asajeffrey

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