Allow rgbgfx -c embedded to accept embedded palettes with extra unused colors

[Rangi42]

Continuing discussion from #1062:

Currently rgbgfx -c embedded expects the PNG's indexed PLTE chunk to define exactly four colors. Some users may have PNGs with more than that (e.g. 256 colors), but only use colors 0-3. Arguably this should be a supported use case.

[ISSOtm]

No, because they may be expecting the extra colours to end up in the generated palette even if they aren't used. Ignoring extra colours if both -n and -s are specified, maybe.

[aaaaaa123456789]

How would that be a reasonable expectation when the palette is four colors?

[ISSOtm]

If it's four colours, then they would be used, and emitted in any order unless -c is passed. -c is the opt-in for "actually, I manipulate colours by index, so it's important that you respect this". Otherwise, they just want to display something, so they would just copy the binary palette data to CRAM.

Grayscale sorting makes more sense by default, since on DMG, palettes are a single byte—not something you'd want to INCBIN—so sorting in a way compatible with the common $E4 palette is a good default.

[aaaaaa123456789]

I was talking about the GBC palette. How would it be reasonable to expect palette data to contain more than four colors (even if the image does) when the GBC palettes can only contain four colors?

[pinobatch]

Test case: testcase2.png has 8 colors and uses the first 4. testcase3.png has 4 colors. Converting them with -c embedded should produce the same character data.

[ISSOtm]

I was talking about the GBC palette. How would it be reasonable to expect palette data to contain more than four colors (even if the image does) when the GBC palettes can only contain four colors?

If it contains 8, it means 2 palettes of 4 colours, for example.

[pinobatch]

Though it'd be impossible to actually have pixels using those additional 4-color subpalettes without -c modulo as described in #1067.

[Rangi42]

This is the same non-issue as #1074: -c embedded does accept embedded palettes with extra unused colors, and just ignores them.