Don't wait for slow imagery before rendering a tile.

As long as the imagery's parent tile is available, we can use that instead
of waiting.  This lets us show faster imagery sooner rather than waiting
for the slowest imagery attached to a tile.
Add a note for myself for later.
Refactor imagery loading.

This fixes the crash that previously occurred in this branch when zooming
in past the available imagery, and makes the code much less insane.

At this point performance appears to be outstanding, but memory usage is
higher than before.  I'm not sure why yet.
Fix layer manipulation.
Remove debug code and unused property.
Fix test failures.
Merge remote-tracking branch 'origin/master' into noWait
Update CHANGES.md.
Merge pull request #851 from AnalyticalGraphicsInc/noWait

Don't wait for all imagery attached to a tile to be loaded before rendering the tile