More robust computation of tile edge planes.
Temporarily disable horizon culling.

It's not working correctly for the root tiles in Canary.
Fix and re-enable horizon culling.
Clean up plane computation.
Update CHANGES.md.
Merge remote-tracking branch 'origin/master' into tilePlanes

Conflicts:
	CHANGES.md
Merge pull request #1387 from AnalyticalGraphicsInc/tilePlanes

Fix tile plane and horizon culling problems in Chrome Canary