When the frontend preprocessor finds "#pragma inject_preset_code" it will search for a file named, and in the same path, as the loaded preset, but with the extension ".inc", and, if found, will "include" in the shader its content.
The use case is to allow presets to change "static" shader features.
This commit does not change the default behaviour of the existing shaders or presets; if a shader wants to take advantage of the feature it has to use "#pragma inject_preset_code" directive.

… a .inc file