Define ToDIP() in wxHAVE_DPI_INDEPENDENT_PIXELS case too

Previously any code using this function wouldn't link under GTK+ 3 or OS X
where wxHAVE_DPI_INDEPENDENT_PIXELS are defined.

Closes #17310.

include/wx/window.h

@@ -1994,7 +1994,9 @@ inline wxWindow *wxWindowBase::GetGrandParent() const
 
 #ifdef wxHAVE_DPI_INDEPENDENT_PIXELS
 
-// FromDIP() becomes trivial in this case, so make it inline to avoid overhead.
+// FromDIP() and ToDIP() become trivial in this case, so make them inline to
+// avoid any overhead.
+
 /* static */
 inline wxSize
 wxWindowBase::FromDIP(const wxSize& sz, const wxWindowBase* WXUNUSED(w))
@@ -2002,6 +2004,13 @@ wxWindowBase::FromDIP(const wxSize& sz, const wxWindowBase* WXUNUSED(w))
     return sz;
 }
 
+/* static */
+inline wxSize
+wxWindowBase::ToDIP(const wxSize& sz, const wxWindowBase* WXUNUSED(w))
+{
+    return sz;
+}
+
 #endif // wxHAVE_DPI_INDEPENDENT_PIXELS
 
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