Blender cannot be told to only allow absolute path for an RNA property
(of type `string`, subtype `dir_path`), so as a workaround the final
`render_output_path` is now using `bpy.path.abspath()` to make the path
absolute.
This has as advantage that the render output path can be defined by artists
as a blendfile-relative path, and that it'll be resolved when submitting
the blend file.
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