distort-stl: wip fixing edge mismatch bug

distort-stl: wip fixing edge mismatch bug

807ba5b3024fb82823d2f3fb9dafa1702470e56a
"poster-tube-lid: Makefile: get set-fa argument right"

was wrong, because it's not permitted to have a vertex of one triangle
in the middle of the edge of an adjacent triangle.

What we should be doing is splitting *edges*.  If we do that based
only on the properties of the *edge*, and get the same midpoint
coordinate each time, then the result will be consistent when we
process the "same" edge as part of different triangles.  So we don't
need to memoise everything.

Right now this is just the initial cut and does not actually run.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>