Feature Request: Ability to supply GLSLang the list of available extensions

Based on the question in #2594, and given in Godot we compile shaders in run-time instead of shipping them with the game (as it is a general purpose game engine), it would be immensely useful for us to be able to pass GLSLang (at least via API) the list of extensions that it should enable.

While I understand this can be made with our own API as macros, it completely beats the purpose of GLSL providing this mechanism and making it available in Vulkan. As far as I understand, GLSLang just assumes all extensions are enabled based on the base SPIR-V version they support, so we would like to make this information be faithful to what the hardware actually supports at the time of the shader being compiled.

I suppose this would be done by just passing GLSLang an optional list of const char* strings with the available extensions.

Start by reviewing the GLSLang API and the context from issue #2594, then trace how shader compilation currently determines enabled extensions. Define an optional API input for the available extension names and verify that compilation uses only the supplied hardware-supported extensions.

Written by the indexing model from the issue text.