render map to PNG format

[kostmo]

Towards #1415.

Uses

This capability could be used to quickly iterate on DSL world descriptions, e.g. when tuning noise parameters.

Implementation notes

• For the hard-coded ANSI terminal color names, I chose RGB triples that matched my own terminal settings. This means that a rendered PNG might not exactly match one's own terminal colors.
• Blank terrain corresponds to a transparent pixel.
• Implemented parse-time validation of attr references. Previously, referencing a nonexistent attr by an entity would fail silently at runtime.
• Normalization: strings like "rock" now only exist once; the string is shared via toplevel variable definitions
• Entities and terrain have TUI-independent color definitions from which VTY Attrs are derived, but all TUI user-interface colors are defined only as VTY Attrs.

Demos

Each pixel in the output image correponds to one world cell. To enlarge, can use imagemagick:

stack run -- map data/scenarios/classic.yaml --seed 0 --png -w 300 -h 200 && convert output.png -scale 800% out2.png

stack run -- map data/scenarios/Challenges/bridge-building.yaml --png && convert output.png -scale 800% out2.png

[byorgey]

FYI, I'm still in the middle of reviewing this PR, but even before I'm finished it's worth noting that this PR somehow changes all the colors in my terminal and it looks really bad:

For comparison, this is what it looks like on the main branch:

[kostmo]

PR somehow changes all the colors in my terminal

I see... I have a theory that the ANSI color constants cannot be reproduced with RGB equivalents. I will try creating a parallel "vty-independent" enum that will map back to the vty constants for TUI rendering.

[kostmo]

Ah, fixed the terrain color, at least, with 42db654.

[byorgey]

Hmm, that's better, but the water and tree colors still look strange to me:

[kostmo]

Ok, have now fixed trees/water (via introducing NamedColor) and have fixed PNG colors by round-tripping through the terminal colorspace (i.e. color240CodeToRGB . rgbColorToColor240).

[byorgey]

This is super cool! Definitely a nice feature to be able to render the world to a PNG, and I think I will make heavy use of it when I get back around to stuff like adding cities and so on to the "classic" world.