[FEATURE] sheet_converter should try to create a single palette

As of `0.2.1`, `sheet_converter` is creating a new palette for each sprite.

This was first introduced in `0.1.4`, as an improvement over generating a single palette for the whole sheet (which may include separator pixels, messing with the obtained palette):
- #9

A better implementation may involve extracting a single palette from combining all sprites in a single image (only after cropping, ensuring no separators are picked up in the palette)