Factor out duplicated code for bitmap scaling in wxArtProvider.

Move duplicated code responsible for rescaling bitmaps in wxArtProvider to the
shared wxArtPrvider::RescaleBitmap() method.

Additionally, make this new method work even in wxUSE_IMAGE==0 case.

See #17071.

See https://github.com/wxWidgets/wxWidgets/pull/64.

src/generic/datavgen.cpp

@@ -570,7 +570,7 @@ public:
     void Resort();
 
 private:
-    wxDataViewMainWindow *m_window;
+    wxDataViewMainWindow * const m_window;
     wxDataViewTreeNode  *m_parent;
 
     // Corresponding model item.