Add a page on variable rate shading (VRS)

[Calinou]

Marked as draft, as I haven't been able to use VRS successfully so far (despite godotengine/godot#68710). It doesn't give me the expected visual result, with 2D rendering and geometry edges also being affected when they shouldn't. Getting VRS to work correctly is needed so I can create comparison screenshots and performance figures.

Edit: I got VRS working on NVIDIA, so I was able to add screenshots and a performance comparison.

Nonetheless, the text portion of the documentation is ready for review.

Testing project: test_vrs.zip

[Calinou]

This PR is now ready for review. Changes since the last revision:

• Mention VRS being usable with any antialiasing technique.
• Mention mobile GPUs that support VRS.
• Add screenshots of textured and untextured scene, with and without VRS.
• Add performance comparison at various viewport resolutions.
• Add a note about the Compatibility rendering method not supporting VRS.
• Add tip about mipmap LOD bias to reduce blurriness when enabling VRS.

[BastiaanOlij]

This looks great to me, did make me realize we should expose some of the GPU capabilities through the render server, it would be nice to be able to query in code whether things like VRS, Multiview, compute shaders, etc. are supported by the platform so the developer can adjust accordingly.

[YuriSizov]

Needs a rebase

[YuriSizov]

Thanks!