Allow fixed maps

[serin-delaunay]

When making images demonstrating what can be done with Wangscape, it would be useful to generate pictures of the same terrain map rendered with different tilesets. That would require one of these two options:

• A key which saves the currently displayed terrain map to a file, and a command line option to load a terrain map file. It should be in a human-editable format.
• A command-line option to choose the random seed used to generate the first map, and a key to display the current map's random seed.

The first option would be slightly more effort, but would be more flexible. Terrain short names are not limited to 1 character, and may be ambiguous without .s interspersed.

[serin-delaunay]

For the file-based option, rather than interspersing with .s I would suggest a format with fixed-width cells. Whitespace is disallowed in terrain IDs, so short IDs can be padded to fill the width. The cell width could be specified in the file header like this:

1
cccc
cggc
cgvc
cccc

(three terrain types: c, g, v)

2
 c c c c
 clglp c
 clp v c
 c c c c

(four terrain types: c, lg, lp, v)

[serin-delaunay]

#23 uses this format:

{
  "size":[2,2],
  "map":[
    ["g","s","v","g"],
    ["g","s","v","g"],
    ["g","s","v","g"],
    ["g","s","v","g"]
  ]
}

Note that the numbers of rows and columns in the map data are each two greater than the given size - the extra terrains at the edge are required to make a scene with the given number of correctly aligned tiles.