Add --screenshot-metadata CLI option

Hi, I would really love a CLI option where I can specify the metadata which is written to a screenshot PNG. This would be an addition to the --screenshot-template option, but would store the metadata in the file not in the files name. In that way, you could really use the %P (path) placeholder, which is normally not usable here, because test-[%P] would be translated to test-[/home/user/test.png].png which is not a valid filename.

Example:

mpv --screenshot-metadata "filename:%f" \
    --screenshot-metadata "path:%P" \
    --screenshot-metadata "position:%fc"

So that pngmeta would print the following output:

$ pngmeta ~/test.png
filename: test.mp4
path: file:////home/user/test.mp4
position: 5835467

Start by tracing mpv's CLI handling for --screenshot-template and the screenshot-writing path. Check how PNG output is currently produced, then define how repeated --screenshot-metadata values map to PNG metadata while preserving the example placeholders. Done means the option accepts the shown inputs and pngmeta can read the resulting filename, path, and position values.