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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---
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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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components/layout_2020/Cargo.toml

@@ -44,7 +44,7 @@ servo_url = { path = "../url" }
 style = { path = "../style", features = ["servo", "servo-layout-2020"] }
 style_traits = { path = "../style_traits" }
 unicode-script = { workspace = true }
-webrender_api = { git = "https://github.com/servo/webrender" }
+webrender_api = { workspace = true }
 
 [dev-dependencies]
 lazy_static = { workspace = true }