Realtime changes overhaul

[dylanebert]

This is a major overhaul that adds support for multiple objects, real-time updates, and intersection testing.

Scene

• The Scene class now consists of Object3D objects.
• Camera and Splat are now types of Object3D. The refer to CameraData and SplatData respectively.
• SplatData now contains the positions, rotations, scales, and colors that previously composed the scene representation.

Renderer

• The WebGLRenderer is now simplified to contain a modular list of shader programs.
• The new ShaderProgram class containerizes rendering logic. This includes the RenderProgram, which is responsible for rendering splats.
• There is now a DataWorker that asynchronously packs data into a render texture.

Intersections

• There is now an IntersectionTester class that can be used for intersection testing.
• The SortWorker now stores the 15x15 screen-space chunk ID of each point, improving intersection testing speed.