fix: missing value in CIELAB lookup table

fix: missing value in CIELAB lookup table

This lookup table contains an approximation of the visible gamut of the CIELAB
color space.¹ While scaling to, e.g., Standard RGB is not linear, it is
continuous. Therefore it follows that if (L=5,a=22,b=6) is visible and
(L=5,a=22,b=8) is visible, (L=5,a=22,b=7) is also visible. This was presumably
an accidental deletion committed in pre-version-control days. Fixes #1974.

  ¹ https://en.wikipedia.org/wiki/CIELAB_color_space