Add Pixmap::take_demultiplied to get raw, demultiplied pixel data

[JustForFun88]

This PR introduces a new consuming method, Pixmap::take_demultiplied, which returns a Vec<u8> containing straight alpha RGBA data.

The new method simply extracts the existing demultiplication loop from PixmapRef::encode_png and makes it public.

Motivation

Currently, the logic to convert from the library's internal premultiplied alpha format to straight alpha is hidden inside the encode_png function. To obtain raw, demultiplied pixel data for other purposes (e.g., interfacing with custom image processing code), users would have to re-implement this logic using PremultipliedColorU8::from_rgba, which is inefficient since PremultipliedColorU8::from_rgba_unchecked is private, or resort to manipulating the underlying slice directly with something like container.chunks_exact_mut(4).

[RazrFalcon]

Looks good to me.

[LaurenzV]

Will be merged after we've fixed the CI issue.