[NTG-context] Shading transparency / missing linear_shade variant

[Vit Zyka]

> Furthermore, the shading variants for linear_shade do not correspond
> with those listed in the metafun manual, page 180. Variant 2 is missing
> and behaves like variant 3 should.

Not so exactly. The today reality is a bit richer than that in the 
Metafun manual. One can choose from 8 directions:

number   direction
1        NE
2        NW
3        SW
4        SE
5        E
6        N
7        W
8        S
other    E

Vit Zyka