Misc graphics cleanup

* Added `cull` option to `Primitive` constructor.
* The `translation` parameter to `Matrix4.fromRotationTranslation` now defaults to `Cartesian3.ZERO`
* Fixed some code examples in the reference doc.
* Fixed off-by-one error in `throttleRequestByServer`.
* Added ability to set `vec3` and `vec4` uniforms using Cesium's `Color` type.
* Initial infrastructure to allow appearances to have uniforms.
* Clean up use of `renderForSpecs` in tests.