[mapping] allow points at the same x value (fixes bug #13617)
[mapping] allow points at the same x value (fixes bug #13617)

svn:9a0661a51fa546353b2044a64a194520c6caa5ad@378

achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint