• Bug 1579235 - Part 8 - Remove overlay tiles, they can be alpha tiles …

    …instead. r=nical
    
    A previous patch in this series introduced overlay tiles. However,
    now that native surfaces exist for for the opaque and alpha tiles
    within a slice, we can remove the overlay tiles array and add
    these special tiles to the alpha surface.
    
    Differential Revision: https://phabricator.services.mozilla.com/D64899
    
    [ghsync] From https://hg.mozilla.org/mozilla-central/rev/38b7f4761f5041a0e3b2326c2be37f98da509e21
    gw3583 authored and moz-gfx committed Mar 2, 2020