Add ability to apply watermarks to downloaded images

[TheFleetCommander]

Apply watermarks to content downloaded from Malmal using the Gallery tool.

The intention here is to allow users to better enforce ownership over their artwork. We can discuss different implementations, but here are some initial ideas to better illustrate what this issue is requesting:

• Automatically apply watermarks to downloads that the logged-in user did not contribute to
• Allow users to opt into a user-setting that automatically applies watermarks whenever another user downloads a piece that this user contributed to (ie. whenever the logged-in user is not downloading their own work)

[lucasmerlin]

How would we decide if a user contributed a meaningful amount to the selected area? Checking the amount of strokes they made on the relevant tiles is unreliable because it might include stokes not made in the selected area (that is a problem on HP right now). I might be able to count only strokes in that area. But what if someone uses a transparent stroke to draw over everything everywhere so their watermark is always shown? Calculating the area that is actually visible would be really complicated and computationally intensive.

I think a watermark would be nice though, might also help grow malmal a bit, since it could also show a link to the board or to malmal at least. Then at least people could go there and see who actually drew it with the inspect tool.