Multiple render passes are needed for supporting shadow mapping, and
this is a huge step towards breaking the Vulkan render free of Quake,
and hopefully will lead the way for breaking the GL renderers free as
well.
After getting lights even vaguely working for alias models, I realized
that it just wasn't going to be feasible to do nice lighting with
forward rendering. This gets the bulk of the work done for deferred
rendering, but still need to sort out the shaders before any real
testing can be done.
