Use KHR_parallel_shader_compile when available

This enables (multiple) shaders to be compiled on a separate thread. To fully take advantage of this, Cesium's renderer would need to be restructured a bit so it doesn't block right away waiting for the results of shader compile/link.

https://www.khronos.org/registry/webgl/extensions/KHR_parallel_shader_compile/

https://www.khronos.org/webgl/public-mailing-list/public_webgl/1811/msg00008.php

Start by reviewing Cesium's renderer flow for shader compilation and linking, then read the KHR_parallel_shader_compile extension specification and linked discussion. Done means the extension is used when available and shader compilation no longer blocks immediately while waiting for results.

Written by the indexing model from the issue text.