3FRDecoder: if present, read black/while level from raw
3FRDecoder: if present, read black/while level from raw

Refs. https://github.com/darktable-org/darktable/issues/5305#issuecomment-641334596

darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed