First cut of a general quadtree primitive.
Much better handling of async geometry.
Closer to working ExtentTileProvider in 2D/CV.
Merge remote-tracking branch 'origin/master' into tiledGeometry

Conflicts:
	Source/Scene/GlobeSurface.js
Adjust for breaking changes in Cesium.
Extent -> Rectangle, remove unused code.
Remove more unused code.
WIP build terrain rendering on QuadtreePrimitive.

Doesn't work at all yet.
Semi-working terrain rendering based on QuadtreePrimitive.
Merge remote-tracking branch 'origin/master' into quadtreePrimitive

Conflicts:
	Source/Scene/Globe.js
	Source/Widgets/Viewer/Viewer.js
Merge remote-tracking branch 'origin/moveTerrainToCore' into quadtreePrimitive

Conflicts:
	Source/Core/GeographicTilingScheme.js
	Source/Core/TilingScheme.js
	Source/Scene/Globe.js
Get back to a semi-working state.
Fix the most serious rendering problems.
Clean up children properly.
Remove unnecessary credit-related functionality.
Merge remote-tracking branch 'origin/master' into quadtreePrimitive
Fix terrain switching and wireframe mode.
Fix layer manipulation.
Fix water, lighting, data propagation, command pooling.
Sort commands by number of textures.