Implements T78397
Extends the functionality of pack islands operator to allow packing UVs
to either the closest or active UDIM tile.
This provides 2 new options for packing UVs :
* Closest UDIM: Selected UVs will be packed to the UDIM tile they were
placed on. If not present on a valid UDIM tile, the UVs will be packed
to the closest UDIM in UV space
* Active UDIM: Selected UVs will be packed to the active UDIM image tile
In case, no image is present in the UV editor, then UVs will be packed
to the tile on the UDIM grid where the 2D cursor is located.
Reviewed By: campbellbarton
Maniphest Tasks: T78397
Ref D12680
a285299ebb