Splitting interface stages based on the interpolation of its
attributes.
Then naming convention that have been used:
* use `interp` as instance name when using smooth interpolation
* use `interp_noperspective` when using no perpective interpolation
* use `interp_flat` when using flat interpolation
The same suffix will be added to the struct names.
The naming convention will be added to the GLSL code style and
applied to other shaders as well.
Pull Request: https://projects.blender.org/blender/blender/pulls/111210
53396816e4