Converting from slp to png seems to lose some of the original information, which pixels are part of the outline, or which are real colours, or player colours.

For use in opengl or vulkan, we could save multiple layers per converted slp file:
1: Sprite plain colours as RGB
2: Player colours: mono (later replaced in shader, with player colour)
3: Outlines: mono (later replaced in shader, with player colour)

Maybe use a different format (can png save multiple layers in one file?)
or create a new format, probably just some container for png, and save to different files in conversion.