iteration's computed du/dv values if they're ready; this improves convergence,
but is hard to parallelize. On the GPU, we render to the same texture as
we render from; as per the OpenGL spec, this will give us undefined behavior
on read (since our read/write sets are neither identical nor disjoint),
iteration's computed du/dv values if they're ready; this improves convergence,
but is hard to parallelize. On the GPU, we render to the same texture as
we render from; as per the OpenGL spec, this will give us undefined behavior
on read (since our read/write sets are neither identical nor disjoint),