Support transparent color at non-zero index

[suchipi]

Right now, when using indexed color in Aseprite, we only support the color at index zero being the transparent color. That's the default, but it's user-configurable.

Technically, this fix needs to happen in ase-parser and then aseprite-loader, but I'm tracking it here.

The indexed color transparent index is stored in the header: https://github.com/aseprite/aseprite/blob/master/docs/ase-file-specs.md