Cache wxDisplayImpl object for faster access

Avoid a heap allocation on every wxDisplay creation by caching the
wxDisplayImpl objects once they're created.

Currently we never invalidate the cache, but we should add a way to do
it in the future.

This speeds up wxDisplay::GetGeometry() benchmark by a factor of 4.

src/common/dpycmn.cpp

@@ -118,12 +118,7 @@ wxDisplay::wxDisplay(unsigned n)
     wxASSERT_MSG( n == 0 || n < GetCount(),
                     wxT("An invalid index was passed to wxDisplay") );
 
-    m_impl = Factory().CreateDisplay(n);
-}
-
-wxDisplay::~wxDisplay()
-{
-    delete m_impl;
+    m_impl = Factory().GetDisplay(n);
 }
 
 // ----------------------------------------------------------------------------
@@ -230,6 +225,15 @@ bool wxDisplay::ChangeMode(const wxVideoMode& mode)
 // wxDisplayFactory implementation
 // ============================================================================
 
+wxDisplayFactory::~wxDisplayFactory()
+{
+    for ( size_t n = 0; n < m_impls.size(); ++n )
+    {
+        // It can be null, that's ok.
+        delete m_impls[n];
+    }
+}
+
 int wxDisplayFactory::GetFromWindow(const wxWindow *window)
 {
     // consider that the window belongs to the display containing its centre