Make the generator generate calls for the correct glBindAttribLocation()
calls.
Usually this was done correctly, but we can't rely on it. So do it
explicitly.
A variation is a #define/specialization constant that every shader can
use to specialize itself as it sees fit.
This commit adds the infrastrcture, future commits will add
implementations.
This shader can take over from the ubershader. And it can be used
instead of launching the ubershader when no offscreens are necessary.
Also includes an optimization that uses the colorize shader when
appropriate.
