babl: make LUT from 2bpp to 4bpp totally generic.

babl: make LUT from 2bpp to 4bpp totally generic.

The deleting of the first byte when creating the LUT, then copying the
first byte from source to destination, worked for cases such as "YA u8" to
"RGBA u8" but it would not work for say:

* "Y half" to anything: in such a case, deleting the first byte in LUT's
  content, when creating it, we are basically deleting source's color
  information!
* "YA u8" to "Y float": in such a case, when copying the first byte, at
  conversion time, we are destroying the result color (which didn't need
  any copying of alpha channel as there is no alpha channel in the
  result).
* And so on.

In such cases, it's just better to include everything in the LUT, in a
totally model-agnostic way. In some cases, the alpha channel will be
stored in the LUT, in others, not. But it's not really a problem. It
doesn't even make the LUT bigger; if anything, it makes the LUT creation
and the conversion calculus simpler, hence faster.