Auto merge of #18316 - MortimerGoro:rust_webvr_0.8, r=jdm

Update rust-webvr to 0.8

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The new rust-webvr version includes features like multiview, opaque framebuffers and multiple layer rendering support that will land in separate PRs.

It also splits rust-webvr traits to a separate crate. This allows much faster compilations when using local cargo replaces.

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