Rendering performance improvements with WEBGL_multi_draw / WEBGL_multi_draw_instanced

Great summary of these new extensions: https://www.khronos.org/webgl/public-mailing-list/public_webgl/1902/msg00000.php

Basically using these extensions is cleaner and simpler than CesiumJS's current manual batching to reduce CPU overhead for WebGL draw calls, and can allow more flexibility for per-draw-call data - instead of duplicating as vertex attributes or storing in a batch table.

This could affect almost anything including:
* Billboard/label/point batching and instancing
* 3D Tiles batching and instancing
* Add batching for terrain
* Geometry batching

Not a priority at the moment but great research project to see where we can improve performance and/or make things simpler. Note that Cesium would require a fallback / slow path for a long while.

CC #797

No files or tests are named. Begin by locating CesiumJS's current manual batching in billboard, label, point, 3D Tiles, terrain, and geometry paths, then review the linked WebGL extension summary. Done means identifying worthwhile performance or simplicity improvements, measuring the affected paths, and preserving a fallback slow path.

Written by the indexing model from the issue text.