[mapping] fix interpolation special cases
[mapping] fix interpolation special cases

When a mapping curve had two points at the same y value the
interpolation code did return nan there. Return the y value of the
first point instead.

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