Parse ColorMap tag from geotiff

[kylebarron]

See https://github.com/geotiffjs/geotiff.js/blob/4da82f6699b7d416676e1feb379a706019ae03d1/src/rgb.js#L27-L39

export function fromPalette(raster, colorMap) {
  const { width, height } = raster;
  const rgbRaster = new Uint8Array(width * height * 3);
  const greenOffset = colorMap.length / 3;
  const blueOffset = colorMap.length / 3 * 2;
  for (let i = 0, j = 0; i < raster.length; ++i, j += 3) {
    const mapIndex = raster[i];
    rgbRaster[j] = colorMap[mapIndex] / 65536 * 256;
    rgbRaster[j + 1] = colorMap[mapIndex + greenOffset] / 65536 * 256;
    rgbRaster[j + 2] = colorMap[mapIndex + blueOffset] / 65536 * 256;
  }
  return rgbRaster;
}

We should be able to apply that on the GPU