Allow use of newer glsl

I noticed this [commit](https://github.com/sebcrozet/kiss3d/commit/40084c35aaba13a5cd9e99404feed4345758f68d) downgrades glsl.

Is it possible to let the user decide this? Assuming they won't be using the builtin effects and shaders since they wouldn't work but it's a explicit decision.

Start with commit 40084c35aaba13a5cd9e99404feed4345758f68d to understand where the GLSL version is downgraded and how shader selection is configured. Trace the built-in effects and shaders from there, then determine how a user-controlled choice could be exposed while making the incompatibility with built-in shaders explicit. Done means users can intentionally select a newer GLSL version without changing the current default behavior.

Written by the indexing model from the issue text.