Raster mask: relative path

Is your feature request related to a problem? Please describe.

If I add a raster mask and move the mask (.pfm file), the sidecar (.xmp) and the original RAW image into another folder, I'll get errors about unknown raster file since it seems to store the PATH in absolute value instead of relative (or maybe relative to the root folder in the processing configuration ? I tried to decode the xmp file without success, and to be honest I don't see why this root folder is needed).

It also makes it fairly hard to share files to other people, e.g. in https://discuss.pixls.us/ via the play_raw feature, and impossible to relocate my photo library.

Describe the solution you'd like

Just store in the sidecar the relative path to the file.

Alternatives

No solution found beside manually fixing all files which is just not possible in a reasonable time, e.g. when moving a whole photo library.

Additional context

What is the motivation behind the current weird mechanism based on a root folder? A similar (but more general) feature was asked in https://github.com/darktable-org/darktable/issues/19829

Start by reproducing the raster-mask relocation case with the .pfm file, .xmp sidecar, and RAW image in a moved folder, then investigate how the sidecar records the path and uses the processing configuration root. Read issue 19829 for related context; done means the moved files resolve through a relative path without manual edits.