WIP Implement use of PS command shfill into grdview (and maybe elsewhere)

[remkos]

Description of proposed changes

As discussed in Oslo, the PostScript shfill command (first introduced in PostScript 3 in 1999) would be quite useful for draping surfaces as in grdview. The biggest advantages would be:

• Shorter code
• Automatic colour interpolation between the colours defined on the vertices (we can now only use a constant colour for each defined triangle).

There may be some other opportunities as well, like plotting 3D spheres or ellipses.

Regarding shfill implementation in grdview, I have come to the following observations based on reading the PostScript Language Reference Manual, third edition, pp. 261-287

• One way to implement it is by "type 5 shading", i.e. using a pseudorectangular lattice. This would be relatively straightforward, since we are anyway mapping a rectangular grid of vertices, but it would not work when that grid includes holes (NaNs). Afaik grdview now moves around those, since a NaN for the z-coordinate cannot be mapped onto the plot. A NaN value for the draped grid would simply lead to it being assigned the appropriate NaN colour, so that would still work, although the automatic colour interpolation would become a mess.
• The second way of implementation is by "type 4 shading", i.e. using a freeform series of triangles. This implementation is closer to the current one in grdview, but will be a bit more complex to implement.
• Either way, the input can be encoded as binary (we would probably then run it through compression and ascii-encoding as we do images), which can also provide quite some savings in size. I would still first implement it through ascii arrays though to see how well it works.
• One thing that we cannot port from the current implementation, is the opportunity to plot both a surface (draped grid) and a mesh (outline of the grid rectangles). So there would be no equivalent to -Qsm. That means we would probably need to remain supporting the original implementation with option -Qs while introducing a new option e.g. -Qf to use shfill. There would be no way to combine a surface plotted and filled using shfill with a mesh.

Reminders

• [ ] Make sure that your code follows our style. Use the other functions/files as a basis.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Describe changes to function behavior and arguments in a comment below the function declaration.
• [ ] If adding new functionality, add a detailed description to the documentation and/or an example.