CIE: Add "CIE L float"

CIE: Add "CIE L float"

Some of these conversions will be leveraged by gegl:shadows-highlights
which needs to go from "Y float" or "YaA float" to "CIE L float".

The conversion from "RGBA float" was added to aid "YaA float" to
"CIE L float" fishes. They go via:
  "YaA float"     to "RaGaBaA float"
  "RaGaBaA float" to "RGBA float"
  "RGBA float"    to "Y float"
  "Y float"       to "CIE L float"

A direct conversion from "YaA float" to "Y 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 RGB conversions to unassociated alpha.

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

https://bugzilla.gnome.org/show_bug.cgi?id=790111