Support custom palette for images

[orsinium]

Apps may use a custom color palette. We should make it possible to draw images with a custom palette. See uses of DEFAULT_PALETTE in images.rs, we should let users to overwrite it in the config. Both in general for the project and only for specific assets.

[BjoernSchilberg]

That would be great. I got stranded here because I wanted to reimplement my old Go Raylib project [1] for firefly. But this one uses the Sprout Land Asset Pack [2], which uses a different color scheme than Sweetie 16.

[1] https://github.com/BjoernSchilberg/go-sproutlings
[2] https://cupnooble.itch.io/sprout-lands-asset-pack

[orsinium]

You can draw images in the default palette and then replace colors in runtime using set_palette. It's less convenient to draw that way (colors you see in the image editor aren't the same as you'll see in the game) but for now gets the job done.