Added alpha_only argument to getbbox()

[radarhere]

Resolves #7207
Resolves #7122

When saving APNG frames, APNG converts the individual frames to RGB to check if there are any differences. If there aren't, then it is a duplicate frame, and can be combined with the previous one to save space.

Pillow/src/PIL/PngImagePlugin.py

Lines 1140 to 1151 in d1731b3

	delta = ImageChops.subtract_modulo(
	im_frame.convert("RGB"), base_im.convert("RGB")
	)
	bbox = delta.getbbox()
	if (
	not bbox
	and prev_disposal == encoderinfo.get("disposal")
	and prev_blend == encoderinfo.get("blend")
	):
	if isinstance(duration, (list, tuple)):
	previous["encoderinfo"]["duration"] += encoderinfo["duration"]
	continue

However, the issue tries to save RGBA frames that become identical when converted to RGB. So instead, this PR changes the code to convert the frames to RGBA.

Except that then introduces a new problem. If two frames have the same alpha values, then ImageChops.subtract_modulo() returns a image showing that there is 0 alpha difference... when is then interpreted by getbbox() as a fully transparent image, meaning that as per #4454, it ignores whatever RGB information there might be.

So this PR adds an argument to the C getbbox() to allow it to turn off the special treatment of the alpha channel.

[Yay295]

consider_alpha doesn't sound quite right to me. I think alpha_only fits a bit better, or reverse it and call it consider_all_bands.

[radarhere]

I've pushed a commit to rename it to alpha_only.

[Yay295]

It can be a separate change, but should this new parameter be exposed to the public api?

[radarhere]

Ok, I've added an alpha_only argument to the Python getbbox(). This was originally requested in #4849

[gsingh93]

I've been using a local build with this patch for a few weeks now and it's been working well, hopefully we can get this merged soon :)

[radarhere]

Be aware that even if it is merged, Pillow 10.0.0 is not due out until July 1.

[radarhere]

I've added a commit to resolve #7207 - the change involves the same logic presented initially for PngImagePlugin, just this time for GifImagePlugin.

[radarhere]

Pillow 10.0.0 has now been released with this fix.