Bug 1714275 - Establish raster roots for picture cache tiles. r=gfx-reviewers,kvark
Bug 1714275 - Establish raster roots for picture cache tiles. r=gfx-reviewers,kvark

This will allow us to select either rasterizing the tiles with a
scale factor of 1.0 (for high performance mode) or at the current
scale of the pinch-zoom (for high quality mode).

Differential Revision: https://phabricator.services.mozilla.com/D116796

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/737ecd2c9621ded2d09d5717065e6c6de206a0c0

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