Fix screen capture on screens with 24 bit depth and 32 bpp

[alex3d]

Currently on 24 bit depth and 32 bpp screens (e.g. ubuntu 24.04 with intel integrated graphics) screen capture produces image with right dimensions but with solid gray color fill. This happens because readDrawableData reads all pixels as fully transparent with alpha=0.

BurntSushi#36

[jezek]

Thanks for the PR.

I haven't tested this myself, but all the changes are logical and seem legit to me. I trust you you tested it well.

Could you make the changes I requested (and test it again with them), please? After, I think, it'll be ready for merge.

[jezek]

@alex3d Thank you for your contribution. I've tested it, it works.