Make Sandcastle demos prettier

I didn't like the jarring unstyled look of Sandcastle demo buttons and drop downs.
This is my attempt to make them look more like the rest of Cesium.
Update Cesium to use new QuantizedMesh extension format.  In place of using a bitfield identifying the presence of an extension, all extensions must start with a unique id followed by a 32 bit length of the extension, followed by the extension bytes.  This new format allows for clients to skip over extensions that it does not know of or care to unpack.
clean pictures
Eliminate Visualizer overhead

This is a major performance improvement when dealing with a large amount of entities.

1. Eliminated the overhead of non-Geometry visualizers during DataSourceDisplay.update.
2. `EntityCollection` and `CompositeEntityCollection` now include the array of modified entities as the last parameter to their `onCollectionChanged` event.
3. Fixed a bug in `Event` that would cause a crash if an event handler unsubscribed to the event in its callback.
4. Minor performance tweak to `AssociativeArray.set`
5. ran `sortRequires`
Updated unit test datafile defining a terrain tile that includes the oct-encoded extension data using the revised extension format.
Merge remote-tracking branch 'origin/master' into visualizer-overhead

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'agi/master' into quantizedMeshExtensions
Rename private variable to be less confusing.
Add `AssociativeArray.contains`

Use it in `EntityCollection` to make the code clearer.
Added two new unit tests to test QuantizedMesh tiles with unknown extensions defined.  The unknown extensionId is 200, which has not been assigned to any extension.
update geometry and appearances demos
update pictures
update descriptions
Merge branch 'master' into sandcastle-pics
cleanup
Merge pull request #1994 from AnalyticalGraphicsInc/sandcastle-pics

Scrub sandcastle examples
Merge pull request #1993 from abwood/quantizedMeshExtensions

Quantized mesh extensions
Update to joint patch for glTF 0.8
Use contains instead of checking return value of get
Merge pull request #1995 from AnalyticalGraphicsInc/gltf-8-patch

Patch for glTF 0.8
Merge pull request #1987 from AnalyticalGraphicsInc/visualizer-overhead

Eliminate Visualizer overhead
Remove the ability to specify a function for layer properties.

For example, gamma, alpha, hue, etc.