Fix appearance of wxMSW wxToggleButtons with bitmaps in pressed state.

Correct the "pushed" state determination in our own drawn code, it didn't work
for wxToggleButton which doesn't return BST_PUSHED from BM_GETSTATE. But it
does have BM_GETCHECK returning its state directly, so add a new virtual
MSWIsPushed() method and implement it differently for it.

Closes #13755.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78251 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/tglbtn.h

@@ -56,6 +56,8 @@ protected:
 
     virtual WXDWORD MSWGetStyle(long flags, WXDWORD *exstyle = NULL) const;
 
+    virtual bool MSWIsPushed() const;
+
     void Init();
 
     // current state of the button (when owner-drawn)