[Request] Addition aka Linear Dodge blendmode

I need to composite some images together with addition blendmode (also known as Linear Dodge)
I've seen that there is a multiply blendmode, i mean it isn't very different from the addition blendmode and would be a quick thing to add.

Cheers

Start by locating Jimp's existing multiply blendmode implementation and the code path used to composite images. Use that behavior as the reference for adding the requested addition, also called Linear Dodge, and verify that compositing produces the expected result.

Written by the indexing model from the issue text.