convert: per-format dest

[sdfg2]

trafficstars

This may be a workaround for the $format problem with convert, but it would be useful if we could set formats..path independently.

[sampsyo]

Sounds fine. Would you still want this if we fixed the $format issue (#1360)?

[sdfg2]

The more I think about it, the more $dest and $path seem a bit overlapping: if I set $dest to /, I can then use $path for a complete path, no?

[sampsyo]

Yes.

[sdfg2]

Well given there's the overlap, I would suggest either per-format $path or $dest. The former allows for fine grained control over everything in the filename, the latter would allow one to use the default $path. I'm not sure which is better, I think the former, actually, and mentioning $dest can be set to /.