This PR improves the place when shader stages are attached to glPrograms.
Previously it was done when shaders stages where created, in the function
create_shader_stage.
This PR will attach the shader stages inside link program.
Ensuring that create_shader_stage doesn't alter the program, which isn't
clear in its name.
Pull Request: https://projects.blender.org/blender/blender/pulls/117407
18b5b0812b