Return "logical" PPI from wxDisplay::GetPPI() in wxGTK3

Just scale the standard PPI by the scaling factor instead of trying to
compute the actual physical PPI, as the latter is not useful and is
incompatible with the value used by all the other applications.

src/gtk/display.cpp

@@ -350,7 +350,17 @@ double wxDisplayImplGTK::GetScaleFactor() const
 
     return 1.0;
 }
-#endif // GTK+ 3.10
+
+wxSize wxDisplayImplGTK::GetPPI() const
+{
+    // The useful value of PPI is the scaled value of the default PPI and not
+    // the actual PPI computed by dividing the number of pixels by the physical
+    // screen size: this is what everyone else uses, and so we should do it
+    // too.
+    return wxDisplay::GetStdPPI()*GetScaleFactor();
+}
+
+#else // GTK+ < 3.10
 
 wxSize wxDisplayImplGTK::GetPPI() const
 {
@@ -370,6 +380,8 @@ wxSize wxDisplayImplGTK::GetPPI() const
     return ppi;
 }
 
+#endif // GTK+ 3.10
+
 wxSize wxDisplayImplGTK::GetSizeMM() const
 {
     wxSize sizeMM;