image: add cast convience method

a simple wrapper for naming consistency amongst other templated classes

image.hpp

@@ -45,6 +45,7 @@ namespace util { namespace image {
 
         /// allocate and return a buffer with the same contents
         template <typename U = T> buffer<U> clone (void) const;
+        template <typename U> buffer<U> cast (void) const { return clone<U> (); }
 
         buffer<T> downsample (float factor) const;