babl: add matrix re-equalization

babl: add matrix re-equalization

This optimizes the coefficients of the matrix ensuring that a RGB 1.0 1.0 1.0
results in exactly CIE Lab 100.0 0.0 0.0 and that equal R,G,B triples yield 0.0
for CIE a, b. This is achieved by rounding to 16.16 fixed point precision,
which can be exactly represented by IEEE double, and then brute-force jittering
the coefficients +/- 1 for the best solution. This is also the rounding needed
for making the matrix well behaved when used in an ICC profile.