Disable adjustmentlayer support on the raindrop filter.

By rule of thumb, filters that cannot multithread should not
be allowed as adjustment layers, (which are, afaik, multithreaded)
so I am half wodering if we should not somehow hardcode whether or
not adjustment layers are supported based on the availability of
multithread, but regardless, it's best to disable adjustment layers
on this specific filter.

BUG:412522