ScriptFu: fix defaults for script args of type Resource

Fixes resource arg defaults in SF scripts.

Formerly a commit in !1378 (closed). Follows !1725 (merged), the underlying machinery.

Test Cases:

Demo>Sphere (old dialog)
Examples> In Scheme (new dialog)
Render>Lava (gradient)
Decor>Stain
Colors>Map>Set Colormap (palette)
Decor>Slide (font)  fails to be enabled???