Using M_add turns out to be a little awkward, because M_add doesn't multiply with the emission colour. simplepbr needs the fallback texture to have (1, 1, 1, 1) colour, but that results in fully whiting out the model without simplepbr.
Therefore I added a new M_emission slot to 1.10.8, a specific slot that's disabled in the FFP (its specific behaviour can't be implemented easily using texture combiners, I'm afraid). This makes use of that slot.
Using M_add turns out to be a little awkward, because M_add doesn't multiply with the emission colour. simplepbr needs the fallback texture to have (1, 1, 1, 1) colour, but that results in fully whiting out the model without simplepbr.
Therefore I added a new M_emission slot to 1.10.8, a specific slot that's disabled in the FFP (its specific behaviour can't be implemented easily using texture combiners, I'm afraid). This makes use of that slot.
Thoughts on this approach?