gdk: Add api to set drag window hotspot

With GdkDragContext now being in charge of placing the window,
it needs to know about the hotspot to place under the cursor.

docs/reference/gdk/gdk3-sections.txt

@@ -968,6 +968,7 @@ gdk_drag_context_get_source_window
 gdk_drag_context_get_dest_window
 gdk_drag_context_get_protocol
 gdk_drag_context_get_drag_window
+gdk_drag_context_set_hotspot
 
 <SUBSECTION Standard>
 GDK_DRAG_CONTEXT

gdk/gdkdnd.c

@@ -480,3 +480,26 @@ gdk_drag_context_get_drag_window (GdkDragContext *context)
 
   return NULL;
 }
+
+/**
+ * gdk_drag_context_set_hotspot:
+ * @context: a #GdkDragContext
+ * @hot_x: x coordinate of the drag window hotspot
+ * @hot_y: y coordinate of the drag window hotspot
+ *
+ * Sets the position of the drag window that will be kept
+ * under the cursor hotspot. Initially, the hotspot is at the
+ * top left corner of the drag window.
+ *
+ * Since: 3.20
+ */
+void
+gdk_drag_context_set_hotspot (GdkDragContext *context,
+                              gint            hot_x,
+                              gint            hot_y)
+{
+  g_return_if_fail (GDK_IS_DRAG_CONTEXT (context));
+
+  if (GDK_DRAG_CONTEXT_GET_CLASS (context)->set_hotspot)
+    GDK_DRAG_CONTEXT_GET_CLASS (context)->set_hotspot (context, hot_x, hot_y);
+}

gdk/gdkdnd.h

@@ -174,6 +174,11 @@ gboolean        gdk_drag_drop_succeeded (GdkDragContext *context);
 GDK_AVAILABLE_IN_3_20
 GdkWindow      *gdk_drag_context_get_drag_window (GdkDragContext *context);
 
+GDK_AVAILABLE_IN_3_20
+void            gdk_drag_context_set_hotspot (GdkDragContext *context,
+                                              gint            hot_x,
+                                              gint            hot_y);
+
 G_END_DECLS
 
 #endif /* __GDK_DND_H__ */

gdk/gdkdndprivate.h

@@ -63,6 +63,9 @@ struct _GdkDragContextClass {
                                 guint32          time_);
   gboolean    (*drop_status)   (GdkDragContext  *context);
   GdkWindow*  (*get_drag_window) (GdkDragContext *context);
+  void        (*set_hotspot)   (GdkDragContext  *context,
+                                gint             hot_x,
+                                gint             hot_y);
 };
 
 struct _GdkDragContext {