…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