Add a gtk_scrollable_get_border

Add a vfunc to return a non-scrollable border around scrollables.
This would be nicer as a property, but we can't add properties
to an interface without breaking 3rd party implementations, so
make this an optional vfunc, and handle it not being set.

docs/reference/gtk/gtk3-sections.txt

@@ -3096,7 +3096,7 @@ gtk_scrollable_get_hscroll_policy
 gtk_scrollable_set_hscroll_policy
 gtk_scrollable_get_vscroll_policy
 gtk_scrollable_set_vscroll_policy
-
+gtk_scrollable_get_border
 
 <SUBSECTION Standard>
 GtkScrollableInterface

gtk/gtkscrollable.c

@@ -312,3 +312,31 @@ gtk_scrollable_set_vscroll_policy (GtkScrollable       *scrollable,
 
   g_object_set (scrollable, "vscroll-policy", policy, NULL);
 }
+
+/**
+ * gtk_scrollable_get_border:
+ * @scrollable: a #GtkScrollable
+ * @border: return location for the results
+ *
+ * Returns the size of a non-scrolling border around the
+ * outside of the scrollable. An example for this would
+ * be treeview headers. GTK+ can use this information to
+ * display overlayed graphics, like the overshoot indication,
+ * at the right position.
+ *
+ * Returns: %TRUE if @border has been set
+ *
+ * Since: 3.16
+ */
+gboolean
+gtk_scrollable_get_border (GtkScrollable *scrollable,
+                           GtkBorder     *border)
+{
+  g_return_val_if_fail (GTK_IS_SCROLLABLE (scrollable), FALSE);
+  g_return_val_if_fail (border != NULL, FALSE);
+
+  if (GTK_SCROLLABLE_GET_IFACE (scrollable)->get_border)
+    return GTK_SCROLLABLE_GET_IFACE (scrollable)->get_border (scrollable, border);
+
+  return FALSE;
+}

gtk/gtkscrollable.h

@@ -25,6 +25,7 @@
 #include <gdk/gdk.h>
 #include <gtk/gtkenums.h>
 #include <gtk/gtktypes.h>
+#include <gtk/gtkborder.h>
 
 G_BEGIN_DECLS
 
@@ -39,6 +40,9 @@ typedef struct _GtkScrollableInterface GtkScrollableInterface;
 struct _GtkScrollableInterface
 {
   GTypeInterface base_iface;
+
+  gboolean (* get_border) (GtkScrollable *scrollable,
+                           GtkBorder     *border);
 };
 
 /* Public API */
@@ -65,6 +69,10 @@ GDK_AVAILABLE_IN_ALL
 void                 gtk_scrollable_set_vscroll_policy     (GtkScrollable       *scrollable,
 							    GtkScrollablePolicy  policy);
 
+GDK_AVAILABLE_IN_3_16
+gboolean             gtk_scrollable_get_border             (GtkScrollable       *scrollable,
+                                                            GtkBorder           *border);
+
 G_END_DECLS
 
 #endif /* __GTK_SCROLLABLE_H__ */