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.
Unless GSK_GPU_SKIP=gradients is given, we sample every point 4x instead
of 1x. That makes the shader run slower (by roughly a factor of 2.5x)
but it improves quality quite a bit.
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.
