Transparent background watermark

Thumbnails and sample images should get a watermark that is visible in transparent or semi-transparent zones of an image instead of the current black background.

The watermark should be discrete, not disturbing.

Current problems are post mistagging as simple_background and black on black posts ( post #715723 ).

See transparent_background.

Trac #1109, 8 months old.

I am not only talking about changing the background color. It would make little difference. Watermarks are to be recognized. For example. On transparent image, the "danbooru" or "transparent_background" text could be drawn behind it, since it is very unlikely any post would have that.