Improve GS quality with XR Camera

[CedricGuillemet]

fixes #15109

Extract needed value from projection matrix to compute focal shader uniform

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/15193/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15193/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15193/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/15193/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/15193/merge
https://gui.babylonjs.com/?snapshot=refs/pull/15193/merge
https://nme.babylonjs.com/?snapshot=refs/pull/15193/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/15193/merge#BCU1XR#0

[bjsplat]

WebGL2 visualization test reporter:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15193/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU (Experimental)

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/15193/merge/testResults/webgpuplaywright/index.html

[RaananW]

Tested on the quest, works great