Feature Request: Compatibility with Older OpenGL

Currently, Varjo doesn't work with OpenGL 2.1 (GLSL 120).
The most immediate issue is
- Varjo emits `layout (location = n)`, which GLSL 120 doesn't support
- Varjo emits in/out interface block, which GLSL 120 doesn't support

I'm happy to hack Varjo to try to get GLSL 120 working. Is there any hint on
where I could start?