We are currently drawing tiles as separate primitives. This doesn't work well for
masking out edge AA between tiles, since they aren't aware of each other.

The change switches image tiles to be drawn as segments sharing the same header.

Differential Revision: https://phabricator.services.mozilla.com/D19458

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/e4db508a2d78aeabf5900e90f345acd083b3d290