Fix FlatZinc posting for element constraints

The element constraint posting logic for FlatZinc when used without an
offset had an error where the posted propagator used an offest of 1
instead of -1, as should be done for 1-indexed arrays like in
MiniZinc/FlatZinc.

Fixes #109

gecode/flatzinc/registry.cpp

@@ -701,10 +701,10 @@ namespace Gecode { namespace FlatZinc {
       rel(s, selector > 0);
       if (isConstant) {
         IntSharedArray sia = s.arg2intsharedarray(ce[1]);
-        element(s, sia, selector, 1, s.arg2IntVar(ce[2]), s.ann2ipl(ann));
+        element(s, sia, selector, -1, s.arg2IntVar(ce[2]), s.ann2ipl(ann));
       } else {
         IntVarArgs iv = s.arg2intvarargs(ce[1]);
-        element(s, iv, selector, 1, s.arg2IntVar(ce[2]), s.ann2ipl(ann));
+        element(s, iv, selector, -1, s.arg2IntVar(ce[2]), s.ann2ipl(ann));
       }
     }
     void p_array_int_element_offset(FlatZincSpace& s, const ConExpr& ce,

gecode/int/element/pair.cpp

@@ -94,7 +94,7 @@ namespace Gecode { namespace Int { namespace Element {
   template<class View>
   forceinline int
   PairValues<View>::val(void) const {
-    return xv.val()+w*yv.val();
+    return xv.val() + w*yv.val();
   }