Bug 1591752 - FFI for WebRender OS compositor integration: Create WrCompositor struct which implements the webrender::Compositor trait and calls through to virtual methods on RenderCompositor. r=sotaro
Bug 1591752 - FFI for WebRender OS compositor integration: Create WrCompositor struct which implements the webrender::Compositor trait and calls through to virtual methods on RenderCompositor. r=sotaro

This patch was largely written by Sotaro.

This patch also adds an off-by-default pref gfx.webrender.compositor that currently does nothing.

Differential Revision: https://phabricator.services.mozilla.com/D50721

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/69f9b16a76be90025457aa38645c3252420e8e1f

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender