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.
This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.
