What it was doing was casting the input buffer to another TRC, basically
pretending the data was what it was not. In particular, it was casting
linear input buffers to non-linear and all other TRCs to linear.
As was noted in #1958, this was in fact a useful trick, yet it's still a
trick. Basically when it's needed, it's either:
1. because the operation implementation does not work in the technically
correct space. Then the operation code should be fixed;
2. or because several work TRC are valid, then an option should be
proposed by the operation;
3. or for artistic reasons, which people are free to implement as
plug-ins or third-party filters, but it should not be a core GIMP
feature.
Therefore for most cases where it felt needed, the real solution will be
to improve the operations' implementations.
c3ef79b3ef
