Upload external mask in img2img

[A2va]

For now on the mask must be draw with the brush in webui and it's not very praticable. My idea is to draw the mask in a external editor and upload it in webui along with original image.

PR merged #636

[theseus232]

Yes please. Right now the masking function isnt too useful without the ability to use a proper mask.

[matthewfcarlson]

I was wondering if I was crazy for not seeing an option to add a mask.

[alejoar]

I've seen @hlky suggest the usage of photoshop for masking a few times (e.g. Sygil-Dev/stable-diffusion#123)

What I am assuming as masking is making the part of the image I want to mask transparent and export as png. However, img2img seems to be ignoring the transparent parts.

I am not clear on what would be the right steps to mask in a third party editor if not making transparent.