Added rawmode argument to Image.getpalette()

[radarhere]

Resolves #6059

At the moment, getpalette() only returns the palette in RGB mode. This PR adds a rawmode argument that allows the user to specify the mode that the palette shoud be returned as.

I've also allowed the rawmode to be None, in case the user would like the palette returned without transformation.

[CookiePLMonster]

Would it make sense to also allow converting via a .tobytes(encoder, **args) call, much like it is done for Images?

[radarhere]

Just for clarity - you've talking about ImagePalette.tobytes(), and you're suggesting that users be able to call ImagePalette.tobytes(rawmode) (encoders are for images, not palettes).

Bear with me as I try and put this into a coherent thought. I don't think there's an inherit pull for Image.tobytes() and ImagePalette.tobytes() to mirror each other. For one thing, the first argument of Image.tobytes() is "encoder" as you say, which is not something that applies to an ImagePalette. For another, these aren't the only tobytes() methods - there's also ImageCmsProfile.tobytes(), Dib.tobytes(), ImageFileDirectory_v2.tobytes() and Exif.tobytes().

After this PR, If you wanted to convert the data from an ImagePalette to a particular mode, you could

im.putpalette(palette)
im.getpalette(rawmode)

I would think of ImagePalette as being for the analysis and modification of a palette.

I can imagine one wanting to

1. Load an image
2. Modify the palette data in ImagePalette
3. Attach the modified palette to the image proper through load() (or one of the many other methods that call load() internally)
4. Convert the palette data to a certain mode, because you want to write it to file, alongside the image data.

But are you saying you have a scenario where you would like to skip step 3? So you have a situation where you would like to load an image, modify the palette, take a copy of the palette for external purposes, and no longer use the image with it's modified palette?

[CookiePLMonster]

Fair enough - I only suggested .tobytes() for consistency of the API, but your point of encoders not applying to palettes makes sense. I think the current approach is fine then.

But are you saying you have a scenario where you would like to skip step 3? So you have a situation where you would like to load an image, modify the palette, take a copy of the palette for external purposes, and no longer use the image with it's modified palette?

Not sure, I imagine that if there is a need for this the user may simply deep copy the palette. But to be precise, my idea was more along the lines of:

1. Load image
2. Get a palette as RGBA regardless of whether the image has alpha or not
3. Write results to file

However, sounds like a modified getpalette() satisfies that requirement just fine too.

[hugovk]

Thanks!