alternate palette support

[city41]

Enable generating alternate palettes for a given set of tiles. The user could render the set of tiles to a sprite using the alternate palette for things like player 2 palette swaps, stuff like that.

the approach

Each input should have an alternatePaletteImageFiles: string[] key. When a generator first creates the tile with the canvas source, if it sees this key, that tile will also have alternatePaletteSources: Canvas[] set.

"cromAnimations": {
  "inputs": [
    {
        "name": "player",
        "animations": [
            {
              "name": "spawn",
              "imageFile": "./player1_spawn.png",
              "alternatePaletteImageFiles": ["./player2_spawn.png"]
            }
         ]
    }
]

Immediately after main palette generation is done, these alternate palette sources are consulted, using the main generated palettes, to generate the alternate palettes. For each tile, the tile's main Canvas source, the palette it got assigned and the alternate palette sources are cross referenced to generate the alternate palette.

During palette code emit, more metadata about palettes is needed. Currently just the palette data is provided, but also need to specify whether the palette is an alternate, and if so, which tiles it is an alternate for.

[city41]

Possibly a better approach is to provide a "color map" image. it would be x by 2, with the first row being the palette that is used in the image, and the second row being the alternate palette(s). sromcrom would add the alternate palettes into the pool and ensure that the two sets of palettes maintain the same indices.