AnalyticalGraphicsInc/cesium — pull request #442 by kring (2013-01-07 16:16:14+00:00)
Title: Do not unload tiles with transitioning imagery.

From the terrain branch...

Previously, it was possible for tiles to be unloaded even while their imagery was transitioning between states. This caused problems in the terrain branch, and could be causing problems we're unaware of in master as well.