Website-based inpainting

[philpax]

As suggested by @cadaeix.

As an alternative to #51, an action row button can be added that DMs the user with a private link to a webpage (hosted by Exilent) in which they can draw over the image with:

• a mask
• basic drawing tools (circle/square brushes, colours)
• noise

A DM is necessary to associate the user with the link.

This could also be used to create initial images for painting over; i.e. a command /canvas that DMs you a link where you can paint whatever, and then it'll upload it. (Hey, it's easier than trying to draw something on mobile.)