Fix use-after free issues with shader programs

[Terpsichord]

This should fix both use-after-errors mentioned in #53, by making shader::Program own the shader::Library that it uses, and by making shader::Library own the underlying bytes of the compiled shader.

[Meziu]

The only thing I want to correct is the absorption of "Entrypoint" into "Library", while they are supposed to be separate concepts. Afaik, multiple dvlb entries can coexist in a compiled shader binary, even though that is basically never used (and rarely useful) in a real program.

Either way, I'm fine with the changes.

[Terpsichord]

I've updated it so that Library::get now takes ownership of the Library, and added a new get_shared method which uses Rc to allow multiple entrypoints from the same library. Library now uses a new MaybeRc enum to avoid any overhead from Rc in the 99% of cases where it's not needed.

Also while creating the examples, I noticed that there's no method to get the location of a uniform in the geometry shader so I've added that too.