Avoid padding indexed PNG palettes

[Delgan]

Hi.

Just a small feature request. Kind of niche use case so let me know if this is out-of-scope for Pillow.

Basically I'd like to save a PNG with bits=4 but with exactly 5 entries in the palette array. Currently, Pillow writes a 16-entry PLTE chunk regardless of the actual number of palette entries.

Could we imagine a small option such as palette_padding=False (default to True for retro-compatibility) to disable padding?

Note that this is permitted by the PNG specs and that Pillow reads these image well.

[aclark4life]

I've created #10029 for this.

[radarhere]

default to True for retro-compatibility

Could you expand on how retaining the current behaviour would be considered 'compatibility'?
If there's software that can't read the images anymore, then that would be good to know about.
If you're talking about suggestion for how to make files smaller, then why shouldn't we always do that? It seems to me like you're just asking for an expansion on the work of #5330

[aclark4life]

why shouldn't we always do that?

Let's try that unless there is a good reason not to.

[Delgan]

default to True for retro-compatibility

Could you expand on how retaining the current behaviour would be considered 'compatibility'? If there's software that can't read the images anymore, then that would be good to know about. If you're talking about suggestion for how to make files smaller, then why shouldn't we always do that? It seems to me like you're just asking for an expansion on the work of #5330

I didn't realize that omitting bits argument would produce the expected result, sorry about that.

Actually, I'm not aware of any compatibility issue, just assumed the padding was intentional for some reason. The update in #10029, which decouples bits from PLTE, seems perfect.