we had both a class member bandjoin, and an instance member
Vips.Image.bandjoin([i1, i2, i3..])
i1.ibandjoin([i2, i3..])
this was confusing and annoying ... get rid of the class one and just
use bandjoin everywhere, so this is now the way to do it:
i1.bandjoin([i2, i3..])
we used to issue a warning and return early, but this can leave garbage
in the *value pointer, I think
ruby gobject-introspection will walk object props during GC and can see
state inbetween init and build when not all objects have been given a
value ... we don't want these warnings
