Bug 1731136 Part 2: Allow native compositors to handle surface backdrops. r=gw
Bug 1731136 Part 2: Allow native compositors to handle surface backdrops. r=gw

This only allows native compositors to claim this capability, without changing
any compositors to do so. In Part 4, the macOS native compositor claims this
capability in conjunction with adding the code that handles the color layers.

Differential Revision: https://phabricator.services.mozilla.com/D127175

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/7afa3f4b5a8506c44fc44a9cecb79a8b10d97a35

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