Fix wxGLCanvas painting glitch during resize with GTK3

Newly exposed window areas would sometimes not be painted after drag resize.

include/wx/gtk/glcanvas.h

@@ -112,6 +112,9 @@ public:
     // implementation from now on
     virtual void GTKHandleRealized() wxOVERRIDE;
 
+#ifdef __WXGTK3__
+    wxSize m_size;
+#endif
 #if WXWIN_COMPATIBILITY_2_8
     wxGLContext      *m_sharedContext;
     wxGLCanvas       *m_sharedContextOf;

src/gtk/glcanvas.cpp

@@ -18,6 +18,27 @@
 #include "wx/gtk/private/wrapgtk.h"
 #include <gdk/gdkx.h>
 
+#ifdef __WXGTK3__
+extern "C" {
+static gboolean draw(GtkWidget* widget, cairo_t* cr, wxGLCanvas* win)
+{
+    GtkAllocation a;
+    gtk_widget_get_allocation(widget, &a);
+    if (a.width > win->m_size.x || a.height > win->m_size.y)
+    {
+        // GLX buffers are apparently not reliably updated to the new size
+        // before the paint event occurs, resulting in newly exposed window
+        // areas sometimes not being painted at the end of a drag resize.
+        gdk_display_sync(gtk_widget_get_display(widget));
+    }
+    win->m_size.Set(a.width, a.height);
+
+    win->GTKSendPaintEvents(cr);
+    return false;
+}
+}
+#endif // __WXGTK3__
+
 //-----------------------------------------------------------------------------
 // emission hook for "parent-set"
 //-----------------------------------------------------------------------------
@@ -190,6 +211,7 @@ bool wxGLCanvas::Create(wxWindow *parent,
 
     m_nativeSizeEvent = true;
 #ifdef __WXGTK3__
+    m_noExpose = true;
     m_backgroundStyle = wxBG_STYLE_PAINT;
 #endif
 
@@ -203,6 +225,9 @@ bool wxGLCanvas::Create(wxWindow *parent,
     g_signal_add_emission_hook(sig_id, 0, parent_set_hook, this, NULL);
 
     wxWindow::Create( parent, id, pos, size, style, name );
+#ifdef __WXGTK3__
+    g_signal_connect(m_wxwindow, "draw", G_CALLBACK(draw), this);
+#endif
 
     gtk_widget_set_double_buffered(m_wxwindow, false);