This PR aims to do the same as #180, but hopefully without making future WebXR compat more annoying.
get_frustum_corners() is now implemented properly, directly from the projection matrix. I believe it should work with the existing OpenXR code, but I cannot test this with any WebXR implementations with "sheared projection matrices", because I am continuing to doubt whether that even exists.
I also added unit tests, which involved a decent amount of code shuffling regardless. The original implementation from #180 is left in as a control in the test code.
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