agg: watermark

[vbmacher]

It would be really nice if this amazing software could also add a "watermark" image to rendered gif, similar to this terminalizer feature. This is maybe related also to asciinema cast format, but I think it would be possible to add the option as command-line argument, e.g.:

agg  --watermark image.svg,pos=absolute,right=15px,bottom=15px,width=100px,opacity=0.3  example.cast example.gif

Exact command-line would need to be beautified. Nice to have would be to support more than one watermarks.