Instead of randomly having headers here and bodies there and vec4s
floating all over the place, formalize the "finalize" process and make
the shaders immutable at that point, with explicit error checking.

This also gives the shaders explicit signatures, which can be used by
the caller in whichever way they please. This also lays the essentially
groundwork for being able to merge shader passes together: generating
namespaced variables that get explicitly renamed when being merged with
other shaders or when being finalized. (Much like what a compiler does)