Add function to enlarge bbox2d.

include/mapnik/box2d.hpp

@@ -96,6 +96,8 @@ public:
     box2d_type& operator*=(T);
     box2d_type& operator/=(T);
     T operator[](int index) const;
+    box2d_type operator +(T other); //enlarge box by given amount
+    box2d_type& operator +=(T other); //enlarge box by given amount
 
     // compute the bounding box of this one transformed
     box2d_type  operator* (agg::trans_affine const& tr) const;

src/box2d.cpp

@@ -403,6 +403,22 @@ box2d<T>&  box2d<T>::operator+=(box2d<T> const& other)
     return *this;
 }
 
+template <typename T>
+box2d<T> box2d<T>::operator+ (T other)
+{
+    return box2d<T>(minx_ - other, miny_ - other, maxx_ + other, maxy_ + other);
+}
+
+template <typename T>
+box2d<T>& box2d<T>::operator+= (T other)
+{
+    minx_ -= other;
+    miny_ -= other;
+    maxx_ += other;
+    maxy_ += other;
+    return *this;
+}
+
 
 template <typename T>
 box2d<T>& box2d<T>::operator*=(T t)