Make combine_two_pixels() handle the fact that the GPU has limited subpixel interpola...

Make combine_two_pixels() handle the fact that the GPU has limited subpixel interpolation precision.

Currently we only use it to somewhat improve the total_weight estimate
to reduce the error slightly. However, the function can now also return
the total error estimate, which will be useful for knowing when we can
and cannot combine weights with reasonable error.