Dynamic Buffers

_Still in progress_.

Support fast dynamic geometry with appearances. This is the dynamic version of #766 and a generic implementation of the [BillboardCollection](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/BillboardCollection.js)'s buffer management.

**Use cases**
- Add one position to end of a polyline. Add `n` positions to the end. Add `1-n` positions to the end of `m` polylines.
- Change one point in a polyline to track a moving object.
- Update the positions only of all billboards.
- Color-ramp/size based on age of point in polyline. Can probably be done completely in the vertex shader.
- 3D object-editing one geometry in a batch of geometries.
- ...

**Implementation**
- [ ] Automatically pack attributes based on semantics/metadata, i.e., group attributes with the same update frequency together.
- [ ] Automatically compress attributes based on semantics/metadata. #419