Add a simple benchmark of wxDisplaySize() and related functions

This benchmark shows that wxGetDisplaySize() has only minimal overhead
compared to wxDisplaySize(), but wxDisplay().GetGeometry() is almost 3
times slower (under wxGTK).

tests/benchmarks/display.cpp

@@ -0,0 +1,30 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        tests/benchmarks/display.cpp
+// Purpose:     wxDisplay benchmarks
+// Author:      Vadim Zeitlin
+// Created:     2018-09-30
+// Copyright:   (c) 2018 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#include "wx/display.h"
+#include "wx/gdicmn.h"
+
+#include "bench.h"
+
+BENCHMARK_FUNC(DisplaySize)
+{
+    int w, h;
+    wxDisplaySize(&w, &h);
+    return w > 0;
+}
+
+BENCHMARK_FUNC(GetDisplaySize)
+{
+    return wxGetDisplaySize().x > 0;
+}
+
+BENCHMARK_FUNC(DisplayGetGeometry)
+{
+    return wxDisplay().GetGeometry().GetSize().x > 0;
+}