Paste RGBA image over another RGBA image without completely overwriting pixels?

[HyperCodec]

I have an image with a transparent background that I'd like to paste over another, but doing so sets all the pixels layered under that background area to fully transparent instead of just not affecting those pixels.

Is there a part of the API to implement this sort of blending? If not (and not planned to be added) , how might I implement it manually?