Fix #9975 libgimp: missing cases for GimpResource in gimpgpparams

I tested it in another branch with upcoming changes to ScriptFu for binding to resources. Seems to work.

There could be another change that could be done. See the existing comments in the code: /* g_type_is_a() because the core has layer subclasses */ Not sure if that is need for GimpResource also? Whether that is just shorter code?