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