knitout-to-yarns gets checkpoints out of order for back-bed plating

[ixchow]

This code makes a single back-bed stitch with carriers 8 and 9 plated:

;!knitout-2
;;Carriers: 1 2 3 4 5 6 7 8 9 10
in 8
in 9
knit - b1 8 9
out 9
out 8

Trying to convert to yarns with:

$ ./knitout-to-smobj plating-test.knitout plating-test.smobj
$ ./smobj-to-yarns plating-test.smobj ../faces/knitout.sf plating-test.yarns

Produces the output (and error):

Will elaborate faces of smobj 'plating-test.smobj' using yarn paths from library '../faces/knitout.sf' and write resulting yarns to 'plating-test.yarns'
smobj-to-yarns: sm.cpp:3219: void sm::mesh_and_library_to_yarns(const sm::Mesh&, const sm::Library&, sm::Yarns*, std::map<sm::Mesh::FaceEdge, unsigned int>*): Assertion `!is_increasing && "checkpoints should be ordered along yarns"' failed.
Aborted

This works properly with front-bed plating knits of both positive and negative direction.