canvas: fix restore() with existing SMask groups and re-land #12363.#12369

Merged

timvandermeij merged 2 commits intomozilla:masterfrom

emilio:better-cancelation-follow-up

Sep 15, 2020

Commits on Sep 12, 2020

canvas: restore() should reflect that smask groups are finished when stateStack is empty.
emilio
committed
canvas: Properly restore all the remaining items in stateStack in endDrawing.
emilio
committed