Fix T40364: Texture images are not properly displayed in the material preview in BI when using SSS

Blender Internal only uses materials with non-zero user counter for SSS and new
mutable libblock copy was keeping users counter at zero.

Now it sets user counter to 1, which is a bit of arguable decision, but which
also kind of makes sense -- meaning callee owns the copied block.

source/blender/blenkernel/intern/library.c

@@ -829,6 +829,7 @@ void *BKE_libblock_copy_nolib(ID *id, const bool do_action)
 
 	id->newid = idn;
 	idn->flag |= LIB_NEW;
+	idn->us = 1;
 
 	BKE_libblock_copy_data(idn, id, do_action);