Add function for reducing multi-frame animated images to FRAME_DISPOSAL_NONE

[asiekierka]

This is something provided by stb_image and is useful for simplifying the handling of animated images; it's also an option provided by tools like GIMP (as "Unoptimize"). I feel it would be beneficial to have in libplum itself, just like palette reduction-related functions.

[aaaaaa123456789]

It's something I have planned, but I haven't done yet. The same function would also take care of rendering smaller frames onto larger frames, of course, since that's an actual problem.

The most likely implementation of this feature will be a flag to plum_load_image plus a standalone function to make this transformation on an existing image.

[asiekierka]

That'd work for me!