An edge can be marked BM_ELEM_SEAM, which means the UV co-ordinates
on either side of the edge are actually independent, even if they
happen to currently have the same value.
This commit optionally add support for UV Seams when computing islands.
Affects UV sculpt tools, individual origins, UV stitch and changing
UV selection modes etc.
Required for upcoming packing refactor which requires seam support
when computing islands.
Differential Revision: https://developer.blender.org/D15875
caf6225a3d