Added has_transparency_data

[radarhere]

Resolves #7384

Adds a method to determine if an image has transparency data - an alpha channel, an RGBA palette or "transparency" in the image's info directory.

I chose this method name to try and indicate that the image may still be opaque. For example, hopper("RGB").convert("RGBA") has an alpha channel, but has no transparent or translucent pixels.

In doing so, I found a similar check in WebPImagePlugin.

Pillow/src/PIL/WebPImagePlugin.py

Lines 335 to 340 in b723e9e

	alpha = (
	"A" in im.mode
	or "a" in im.mode
	or (im.mode == "P" and "transparency" in im.info)
	)
	im = im.convert("RGBA" if alpha else "RGB")

So I've changed that to use this new method.

[chinery]

Hi @radarhere – thanks for this PR! My only question was going to be whether image palettes support modes other than RGBA, but you've addressed that in your comments to @homm, so from my perspective (of admittedly limited experience), I think it looks great.

[hugovk]

This could also be a property, any preference?

[radarhere]

Sure, that seems like a reasonable idea. I've added a commit to change it to a property.

[hugovk]

Looks good, let's also add to the release notes.

[radarhere]

Sure, I've pushed a commit for that.

[hugovk]

Thank you!