Do not rely on instance shader parameters for recoloring. Instead, make the
material unique when a different recolor is needed. For instance, by duplicating
the cel_shading_recolor_material.tres.

Now the way to have the same recoloring in multiple sprites is to share the same
material.

Townie: have a duplicate copy of the cel shading recolor material. Apply it to all
the parts.

Before, the shader swapped 3 colors: light, medium and dark. The script had a
way to derive the light and dark colors from the medium one. And the key colors
were 3 shades of green.

Go back to the palette-swap shader reference [1] and only change variable names
and comments.

Townie asset:

- Change the colors so skin is yellow and clothes are green.

- Manually paint the contour lines of shapes, to a darker value of the fill
  color. This adds a 4th color to the cel shading, which will be mapped to a
  dark line or a light line, for more readability.

- Export all PNG spritesheets.

CelShadingRecolor:

- Pass the two palettes to the shader.

- Add lists of hair tones and clothes tones. The latter is rather arbitrary and
  could be improved in the future.

- Export buttons to randomize the first 4 colors of the palette for the skin,
  the next 4 for the hair, and the last 4 for the clothes.

[1] https://github.com/KoBeWi/Godot-Palette-Swap-Shader/blob/master/addons/PaletteSwapShader/SimplePaletteSwap.gdshader

Resolve #2204