Deferred shading

[bfops]

The finer points of shading (e.g. texturing and lighting) to a deferred shader don't need to happen for every fragment; they only need to happen for every rendered fragment. Move them to a deferred shader to avoid the extra work.

[bfops]

WIP https://github.com/bfops/playform/tree/deferred-shading

[bfops]

Done-ish, except depth fog (and horribly hacky code), but the performance graphs look exactly the same.