In the latest discussions about curves/hair mesh attachement
information (T95776), it was decided to use UV coordinates to
store where on the mesh each root is. For that, we have to specify
which of the UV map attributes to use for UV lookups.
This property isn't used yet, but it will be shortly when refactoring
the attachement information in the add brush and the to particle
system conversion.
Differential Revision: https://developer.blender.org/D15115
12722bd354