gtk: add gl-area support on win32

On Windows, we don't use the low-level GBM/EGL helpers (no dmabuf etc),
we can turn on GL area support for the rest of rendering.

(fwiw, GDK backend may be either WGL or EGL)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230515132527.1026064-1-marcandre.lureau@redhat.com>

ui/gtk.c

@@ -2443,6 +2443,12 @@ static void early_gtk_display_init(DisplayOptions *opts)
             gtk_use_gl_area = true;
             gtk_gl_area_init();
         } else
+#endif
+#if defined(GDK_WINDOWING_WIN32)
+        if (GDK_IS_WIN32_DISPLAY(gdk_display_get_default())) {
+            gtk_use_gl_area = true;
+            gtk_gl_area_init();
+        } else
 #endif
         {
 #ifdef CONFIG_X11