The picture cache code retains a set of tiles that are currently
off-screen but might be needed again soon, depending on how the
page is scrolled.

However, off-screen tiles were being skipped during draw processing,
which meant that the texture cache request method was not being
called on these tiles. This would often result in the texture
cache eagerly evicting these seemingly unused surface tiles.

This patch re-arranges the occlusion and visibility processing
code for tiles, so that if a tile has been retained in the
picture cache grid, the texture surface is always requested,
even if that tile is currently off-screen. This prevents the
texture cache from evicting tiles that we want to retain for now.

Differential Revision: https://phabricator.services.mozilla.com/D69760

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/c39512f89aedf7c0745137da9c8c09a54f9ab2e8