add valid() method to box2d, and comment subtraction override as it does not have any function currently

bindings/python/mapnik_envelope.cpp

@@ -246,7 +246,7 @@ void export_envelope()
             )
         .def(self == self) // __eq__
         .def(self + self)  // __add__
-        .def(self - self)  // __sub__
+        //.def(self - self)  // __sub__
         .def(self * float()) // __mult__
         .def(float() * self) 
         .def(self / float()) // __div__

include/mapnik/box2d.hpp

@@ -80,6 +80,7 @@ public:
     void re_center(const coord<T,2>& c);
     void init(T x0,T y0,T x1,T y1);
     bool from_string(const std::string& s);
+    bool valid() const;
         
     // define some operators 
     box2d_type& operator+=(box2d_type const& other);

src/box2d.cpp

@@ -348,20 +348,31 @@ bool box2d<T>::from_string(const std::string& s)
     
     return success;
 }
-    
+
+template <typename T>
+#if !defined(__SUNPRO_CC)
+inline
+#endif
+bool box2d<T>::valid() const
+{
+    return (minx_ <= maxx_ && miny_ <= maxy_) ;
+}
+
 template <typename T>
 box2d<T>&  box2d<T>::operator+=(box2d<T> const& other)
 {
     expand_to_include(other);
     return *this;
 }
-    
+
+/*
 template <typename T>    
 box2d<T>& box2d<T>::operator-=(box2d<T> const& other)
 {
     // not sure what to do here. intersect?
     return *this;
 }
+*/
     
 template <typename T>    
 box2d<T>& box2d<T>::operator*=(T t)