babl: don't create LUT for destination formats with premultiplied colors.

babl: don't create LUT for destination formats with premultiplied colors.

This is a different issue than previous commit, but was revealed by
fixing the bug. So in a way, it's a followup.

When the destination format uses premultiplied colors, we would end up
with a LUT where all colors in the table are (0, 0, 0) since we store
them with alpha = 0. We could also generate them with alpha = 1, but
then we'd have to special-case the conversion step, re-multiplying each
pre-multiplied value with the real alpha, etc. So instead, I just bypass
these conversion cases and don't create LUT for these.

For the record, we had this case on GIMP when rendering u8 images to
display, since it would require for instance a LUT from R'G'B'A u8 to
cairo-ARGB32 (Cairo's ARGB32 being pre-multiplied with alpha). So the
whole image would render black (even though, when color-picking colors,
we'd see pixels' colors are not black).

Note that this bug existed before, but was simply hidden by the bug
fixed in the previous commit (since conversion was done twice: first
with LUT, then through usual conversion paths, the second conversion was
hiding the broken LUT conversion).