Allow alpha_composite destination to be negative

[radarhere]

Resolves #5298

The issue argues that since you are able to paste() to negative co-ordinates -

offset.paste(over, (-64, -64))

you should also be able to alpha_composite() to negative co-ordinates -

offset.alpha_composite(over, (-64, -64))

The argument seems reasonable to me.

[hugovk]

Let's also add to release notes