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.
Not only does it makes sense to centralize the setting of viewport and
scissor, but it's actually necessary in order to fix the upside-down
rendering on windows.
While I could reconstruct the position from the screen coords and depth,
this is easier and good enough for now. Reconstruction is an
optimization thing.
Lighting doesn't actually do lights yet, but it's producing pixels.
Translucent seems to be working (2d draw uses it), and compose seems to
be working.
