[glsl-in] Keep around extra linkage variables

WGSL will require this. Note that this still might
cause some issues with multi-entry-point GLSL
that I didn't know how to handle. That is, we will
handle unused builtin inputs but not unused
builtin outputs correctly right now. This is an existing
issue though, not a regression.

This is also provided as an option, but I feel like
the more correct approach is to never strip linkage
variables. We'll see though.