Reposition hook for terrasync.

When re-init-ing the sim, tell TerraSync what's going on (we don't want
to actually reset it, since that would disturb in-flight downloads).
Add an explicit method so it can reset its last lon/lat, and hence do
the initial sync at the new location correctly.

simgear/scene/tsync/terrasync.cxx

@@ -1041,3 +1041,7 @@ bool SGTerraSync::schedulePosition(int lat, int lon)
     return Ok;
 }
 
+void SGTerraSync::reposition()
+{
+    last_lat = last_lon = NOWHERE;
+}

simgear/scene/tsync/terrasync.hxx

@@ -49,6 +49,11 @@ public:
     virtual void unbind();
     virtual void update(double);
 
+    /// notify terrasync that the sim was repositioned, as opposed to
+    /// us travelling in a direction. Avoid last_lat / last_lon blocking
+    /// certain tiles when we reposition.
+    void reposition();
+    
     bool isIdle();
     bool schedulePosition(int lat, int lon);