gdk: Allow passing the start coordinates in drag_begin

Add a variant of gdk_drag_begin that takes the start position
in addition to the device. All backend implementation have been
updated to accept (and ignore) the new arguments.

Subsequent commits will make use of the data in some backends.

gdk/wayland/gdkdnd-wayland.c

@@ -375,7 +375,9 @@ create_dnd_window (GdkScreen *screen)
 GdkDragContext *
 _gdk_wayland_window_drag_begin (GdkWindow *window,
 				GdkDevice *device,
-				GList     *targets)
+				GList     *targets,
+                                gint       x_root,
+                                gint       y_root)
 {
   GdkWaylandDragContext *context_wayland;
   GdkWaylandDisplay *display_wayland;