Use of static var in MapScript mapObj.nextLabel

[dmorissette]

While working on the implementation of label priority, I noticed that the mapObj.nextLabel() method uses a static variable to iterate over labels. This is not safe for multi-thread environments, and does not allow using the function more than once per instance of label cache.

The same code is present in mapscript/swiginc/map.i and in mapscript/php3/mapscript_i.c

What do we want to do to fix it?

  labelCacheMemberObj *nextLabel() {
    static int i=0;

    if(i<self->labelcache.numlabels)
      return &(self->labelcache.labels[i++]);
    else
      return NULL;	
  }

[dmorissette]

Closing as duplicate of #1794.