adjust z0 normal scale

[meetar]

Adjusted the normal-scaling function at low zooms – the effect is eased in through z1 and a very small amount at z2.

Using a custom local test in the differ (new style on the left, old in the middle, differing pixels in red on the right):

By z2 the effect is only barely perceptible in the most extreme slopes, like this one in Greenland:

Past that you see the odd slight difference in a pixel here or there up to around z12:

[meetar]

resolves #198

[nvkelso]

LGTM