Auto merge of #29961 - mrobinson:vendor-webrender, r=atbakhi

Vendor the current version of WebRender

This is a step toward upgrading WebRender, which will be upgraded and
patched in the `third_party` directory. This change vendors the current
private branch of WebRender that we use and adds a `patches` directory
which tracks the changes on top of the upstream WebRender commit
described by third_party/webrender/patches/head.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they do not change behavior.

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