W32: Implement composition check for GDK

Also move DWM function grabbing and make those functions available to all of GDK-Win32.

https://bugzilla.gnome.org/show_bug.cgi?id=727316

gdk/win32/gdkmain-win32.c

@@ -48,6 +48,12 @@ static gboolean gdk_synchronize = FALSE;
 
 static gboolean dummy;
 
+#ifdef HAVE_W32_DWM
+static HMODULE dwmdll = NULL;
+PFN_DwmEnableBlurBehindWindow dwmEnableBlurBehindWindow = NULL;
+PFN_DwmIsCompositionEnabled dwmIsCompositionEnabled = NULL;
+#endif
+
 const GOptionEntry _gdk_windowing_args[] = {
   { "sync", 0, 0, G_OPTION_ARG_NONE, &gdk_synchronize, 
     /* Description of --sync in --help output */              N_("Don't batch GDI requests"), NULL },
@@ -132,6 +138,29 @@ _gdk_win32_windowing_init (void)
   _cf_text_html = RegisterClipboardFormat ("text/html");
 
   _gdk_win32_selection_init ();
+
+/* HAVE_W32_DWM means that we have necessary declarations at compile-time,
+ * but we'd still like to be able to run on XP, so we'll load the functions
+ * we need at runtime.
+ */
+#ifdef HAVE_W32_DWM
+  if (dwmdll == NULL)
+    {
+      dwmdll = LoadLibraryA ("dwmapi.dll");
+      if (dwmdll == NULL)
+        {
+          /* This will make sure that we only try this once */
+          dwmdll = INVALID_HANDLE_VALUE;
+        }
+      else
+        {
+          dwmEnableBlurBehindWindow = (PFN_DwmEnableBlurBehindWindow)
+              GetProcAddress (dwmdll, "DwmEnableBlurBehindWindow");
+          dwmIsCompositionEnabled = (PFN_DwmIsCompositionEnabled)
+              GetProcAddress (dwmdll, "DwmIsCompositionEnabled");
+        }
+    }
+#endif
 }
 
 void