[mapping] simplify linear interpolation formula
[mapping] simplify linear interpolation formula

svn:9a0661a51fa546353b2044a64a194520c6caa5ad@376

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