In Draw compositor mode, tiles are placed into a bucket based on
the tile kind and/or opacity. This simplifies the renderer code
for this mode, which uses the z-buffer to reject opaque tiles.

In native compositor mode, the tiles need to be submitted in the
correct draw order. Opacity is a property of the native surface,
and should not affect the order the tiles are added to the visual
tree of the native compositor.

Differential Revision: https://phabricator.services.mozilla.com/D52933

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/e7837d7a78c67fb122f6f4685eb0e2f06841b621