Support disabling by-channel scaling

[jpsacha]

By default, dcraw computes channel scaling using color matrices read from image metadata. That may not be desirable if you expect minimal raw data modification. In some cases this may actually cause some pixels to saturate.

Support disabling per-channel a fixed multiplier of 1 for all channels, same as command line option -r 1 1 1 1

Most logical place for this is in the white balance options at the Disable white balance option.