Separate entry points when linking multiple GLSL into a single SPIRV

I'm calling it as
```
glslValidator -V -l -o quad.spv quad.vert quad.frag
```
Inspecting the produced SPIR-V shows two entry points named "main". We need to have separate entry points to make those functions available. Passing `-e something` doesn't seem to be doing anything.

Start by reproducing `glslValidator -V -l -o quad.spv quad.vert quad.frag` and inspect the generated SPIR-V entry points. Investigate why `-e something` has no effect; done means linked vertex and fragment shaders expose separate usable entry points instead of two entries both named `main`.

Written by the indexing model from the issue text.