Bug 1669840 Part 3: Include image instance color alpha in check for opaque backdrop. r=gfx-reviewers,lsalzman
Bug 1669840 Part 3: Include image instance color alpha in check for opaque backdrop. r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D114988

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/31dfd306b953a4c5bfc6ca5de51511f2b044c3eb

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