visualizer-view: add helper viewport for scrolling

We can avoid using the inline scrollbar in favor of overlay
scrolling if we bridge the adjustment into the GtkViewport.
This sadly requires us to subclass things to get it just
right.

lib/sp-viewport.h

@@ -0,0 +1,34 @@
+/* sp-viewport.h
+ *
+ * Copyright (C) 2016 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef SP_VIEWPORT_H
+#define SP_VIEWPORT_H
+
+#include <gtk/gtk.h>
+
+G_BEGIN_DECLS
+
+#define SP_TYPE_VIEWPORT (sp_viewport_get_type())
+
+G_DECLARE_FINAL_TYPE (SpViewport, sp_viewport, SP, VIEWPORT, GtkViewport)
+
+GtkWidget *sp_viewport_new (void);
+
+G_END_DECLS
+
+#endif /* SP_VIEWPORT_H */