… drawable array instead of a single drawable.
Instead of expecting a single drawable, GimpImageProcedure's run()
function will now have an array of drawable as parameter.
As a consequence, all existing plug-ins are broken again. I am going to
fix them in the next commit so that this change can be easily reviewed
and examined if needed later.
I only fix the Vala demo plug-in now (or rather, I just use the first
layer in the array for now) because otherwise the build fails.
