Add ncplane_highgradient()

[dankamongmen]

We could work many more shades into our gradients if we used double halfblocks. Why aren't we?

[dankamongmen]

We explicitly supply the EGC used for the gradient, so that doesn't fit with this idea really well. We could just arbitrarily break up FULL BLOCK, but that's unlike any other behavior. I think if we want this, we want to do it as its own function, ncplane_highgradient() or something.

If we do this, we want to update the calls in intro and sliding.

[dankamongmen]

OLD:

NEW:

[dankamongmen]

Look at piece 01 in each, and the improvement is obvious. I've also fixed up intro. Fixed!