default smart pointer type switched to intrusive_ptr

git-svn-id: svn://svn.code.sf.net/p/magicseteditor/code/trunk@337 0fc631ac-6414-0410-93d0-97cfa31319b6

src/gfx/gfx.hpp

@@ -127,7 +127,7 @@ void set_alpha(Image& img, const Image& img_alpha);
 /// An alpha mask is an alpha channel that can be copied to another image
 /** It is created by treating black in the source image as transparent and white (red) as opaque
  */
-class AlphaMask {
+class AlphaMask : public IntrusivePtrBase<AlphaMask> {
   public:
 	AlphaMask(const Image& mask);
 	~AlphaMask();