CIE: Add conversion from "RGBA float" to "CIE Lab float"

CIE: Add conversion from "RGBA float" to "CIE Lab float"

Conversions from "RaGaBaA float" to "CIE Lab float", as seen when
using gegl:shadows-highlights" go via:
  "RaGaBaA float" to "RGBA float"
  "RGBA float"    to "RGB float"
  "RGB float"     to "CIE Lab float"

A direct conversion from "RaGaBaA float" to "CIE Lab float" in simple
C is hindered by the need to check every pixel's alpha value to avoid
dividing by zero. The pipeline stalls make it lose out to the look-up
table and SIMD based conversions to unassociated alpha.

However, we can trivially cut out the second step and still reduce
some memory traffic.

https://bugzilla.gnome.org/show_bug.cgi?id=789695