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.

third_party/webrender/glsl-to-cxx/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "glsl-to-cxx"
+version = "0.1.0"
+license = "MPL-2.0"
+authors = ["The Mozilla Project Developers"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+glsl = "4.0"

third_party/webrender/glsl-to-cxx/README.md

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+A GLSL to C++ translator.
+
+Translates GLSL to vectorized C++. Intended for use with WebRender software backend.

third_party/webrender/glsl-to-cxx/src/main.rs

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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use glsl_to_cxx::translate;
+fn main() {
+    println!("{}", translate(&mut std::env::args()));
+}