qt: preprocess the shaders and support `GL_GOOGLE_include_directive`

This is by no means a proper preprocessor, but it should be more than enough for our needs.

Request review @chub. Feel free to support invoking QSB with defines, but ideally #include should be supported regardless. Maybe this way is better than invoking QSB with defines directly, because as far as I know, Makefile does not support adding files with parameters.

I also noticed that modifying the shaders does not trigger re-compilation with Makefile. I'm not sure what is wrong there, because the dependency rule is there (%.frag.qsb: %.frag and %.vert.qsb: %.vert). This seems to be already an issue, I'm open to suggestions.