Compilation fixes for C++98 compilers in D2D graphics code

Don't use ">>" token in nested templates, this is not allowed in C++98 but
only C++11.

Also don't use "= NULL" on pure virtual functions.

See #16625.

src/msw/graphicsd2d.cpp

@@ -314,10 +314,10 @@ class wxD2DResourceManager;
 class wxD2DManagedObject
 {
 public:
-    virtual void Bind(wxD2DResourceManager* manager) = NULL;
-    virtual void UnBind() = NULL;
-    virtual bool IsBound() = NULL;
-    virtual wxD2DResourceManager* GetManager() = NULL;
+    virtual void Bind(wxD2DResourceManager* manager) = 0;
+    virtual void UnBind() = 0;
+    virtual bool IsBound() = 0;
+    virtual wxD2DResourceManager* GetManager() = 0;
 
     virtual ~wxD2DManagedObject() {};
 };