Static lights are yet to come (so the screen is black most of the time),
but dynamic lights work very nicely (and look very good) despite the
falloff being incorrect.
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.
