Combine TextureAtlas and TextureAtlasBuilder.
Make billboards work with the new texture atlas. Subregions of a texture and labels are still a WIP.
Add destroyTextureAtlas back to billboard collection.
Fix label collection.
Add image sub-regions to billboards.
Fix Sandcastle examples.
Fix Billboard and BillboardCollections tests. Remove TextureAtlasBuilder tests.
Fix remaining tests.
Some clean up and add doc.
Add tests for new functions in TextureAtlas.
Update CHANGES.md.
Cleanup examples.
Update doc code examples.
Tweak example.
Merge branch 'texture-atlas2' into texture-atlas
Clean up BillboardCollection spec.
Initial changes to simplify billboard texture creation.
Fix TextureAtlas tests.
Add Sandcastle.addDefaultToolbarButton

Sandcastle.addDefaultToolbarButton excutes and highlights the toolbar immediately, eliminating the need for breaking out an extra function to call (muddying the code in the process).
Update billboard code to match changes.