COW cleanup patch (1583966):

1. perform deep comparison for the classes for which it makes sense in the
   ports where this wasn't done yet
2. remove (shallow) comparison operators for the classes for which it does
   not make sense (such as wxBitmap)
3. makes wxBitmap use COW on all ports
4. adds wxObject::IsRefTo()
5. centralizes and improves COW docs


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42752 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/object.h

@@ -473,6 +473,9 @@ public:
     // Make sure this object has only one reference
     void UnShare() { AllocExclusive(); }
 
+    // Do a shallow comparison of our referenced data with the given object's refdata
+    bool IsRefTo(const wxObject *p) const { return m_refData == p->m_refData; }
+
 protected:
     // ensure that our data is not shared with anybody else: if we have no
     // data, it is created using CreateRefData() below, if we have shared data