Issue #3471 Unexpected results from luma darken only blend mode
Having the blend mode be perceptual results in the unexpected
result that a color with a lower actual XYZ "Y" value - that's
also clearly visibly darker than the underlying layer - can be
considered as "lighter" than a color with a higher "Y" value.
Reviewer's (Jehan) note: this only changes the default's blend space,
but in fact, we should probably even make the blend space immutable for
these 2 layer modes because the algorithm doesn't make sense in
perceptual (see #3471). We are not doing this right now in order to
not break existing XCF files using these modes in perceptual. We'll have
to figure out later how to have an immutable blend space going forward
while allowing the broken blend space but only for existing XCF images.
