gimpprocedure: Use less strict typechecking

When executing `GimpProcedure.run()`, we perform some type checking
(and sometimes even do more validation) to make sure that plug-ins are
behaving correctly.

The problem with the current implementation was that a plug-in that
accepted for example a single `GimpDrawable` as argument, then trying to
call this with a value of `GIMP_TYPE_CHANNEL` didn't pass the
validation, even though the latter is just a subclass of the former.

For that reason, do a proper typecheck using `g_type_is_a()`.

libgimp/gimpprocedure.c

@@ -1911,7 +1911,7 @@ gimp_procedure_validate_args (GimpProcedure   *procedure,
       GType       arg_type  = G_VALUE_TYPE (arg);
       GType       spec_type = G_PARAM_SPEC_VALUE_TYPE (pspec);
 
-      if (arg_type != spec_type)
+      if (! g_type_is_a (arg_type, spec_type))
         {
           if (return_vals)
             {