Get auto white balance coefficients

[kwilinsi]

Any chance you could add a way to get the auto white balance coefficients after postprocessing? Based on this forum post, it looks like LibRaw supports this with "imgdata.color.pre_mul[] coeffs."

Note that the related issues #12 and #221 are about generating wb coefficients from a gray box (i.e. the -A flag in dcraw). I'm asking about entire-image wb (-a). Also, the above forum post came a year after this was mentioned in #12, so the LibRaw function might be newer.

Also, if it's possible to calculate the wb coefficients without fully postprocessing (e.g. via a separate function), that'd be even better for my use case.