Stable codegen + caching the generated code.

[yujin-wu]

Issue

While working on CSP compliance for plotly, we had the challenge of avoiding codegen in regl. We found that some changes in Regl were necessary to make this possible, in particular making generated code stable.

The changes we needed are in #637 and I believe they are beneficial to regl:

1. Make generated code for the same shader identical

• across runs in any app and construction order
• across all devices

2. Cache for the generated code, which is practical now that a lot of generated code will be identical.
   The PR text has more details.

PR

#637

[yujin-wu]

@rreusser (+@alexcjohnson)

Hey Ricky, would you be able to have a look please? 🙂