core: use full rct signatures if just one input

It is slightly smaller, but requires all rings to have the real
input on the same index, so can only be used for single input txes.

src/cryptonote_core/cryptonote_format_utils.cpp

@@ -656,7 +656,10 @@ namespace cryptonote
       size_t n_total_outs = sources[0].outputs.size(); // only for non-simple rct
       BOOST_FOREACH(const tx_source_entry& src_entr,  sources)
         all_rct_inputs &= !(src_entr.mask == rct::identity());
-      bool use_simple_rct = true; //all_rct_inputs;
+
+      // the non-simple version is slightly smaller, but assumes all real inputs
+      // are on the same index, so can only be used if there just one ring.
+      bool use_simple_rct = sources.size() > 1;
 
       if (!use_simple_rct)
       {