Fix crash with --resource-set-binding [n] (common set form, not per-register form)

--resource-set-binding has a mode which allows per-register assignments of
bindings and descriptor sets on the command line, and another accepting a
single descriptor set value to assign to all variables.

The former worked, but the latter would crash when assigning the values.
This fixes it, and makes the former case a bit more robust against premature
termination of the pre-register values, which must come in (regname,set,binding)
triples.

This also allows the form "--resource-set-binding stage setnum", which was
mentioned in the usage message, but did not parse.

The operation of the per-register form of this option is unchanged.

glslang/MachineIndependent/iomapper.cpp

@@ -404,6 +404,11 @@ struct TDefaultIoResolverBase : public glslang::TIoMapResolver
     {
         if (type.getQualifier().hasSet())
             return type.getQualifier().layoutSet;
+
+        // If a command line or API option requested a single descriptor set, use that (if not overrided by spaceN)
+        if (baseResourceSetBinding.size() == 1)
+            return atoi(baseResourceSetBinding[0].c_str());
+
         return 0;
     }