babl: add R~G~B~ color models and pixel formats

babl: add R~G~B~ color models and pixel formats

From a given BablSpace which defines RGB chromaticities and
per component TRCs, the following are the defined formats:

R'G'B' with the TRCs applied
RGB    linear data

and now a new one

R~G~B~ with the sRGB TRC, this permits any BablSpace to be the
native format of a buffer in an application while having both
a 'perceptual' in R~G~B~ and linear in RGB that is possible
to derive from it. Importantly it is also possible to recover
the babl space used to create the buffer.