Blender Git Loki

Fix RNA's "editable" func regarding IDProps and override.

`RNA_property_overridable_get()` need the original RNA property (i.e.
the actual IDProp in case it is one), not the 'proxy' type property
returned by `rna_ensure_property()` for IDProps.

Makes custom properties of library overrides editable at last, now we
only have to keep them overridden values!