gtk: fix context menu computed location (#5710)

When a tab bar is displayed, the context menu opened with right click is
offset from the cursor.

This was due to using the incorrect coordinate space for describing
where to draw the context menu.


[gtk_popover_set_pointing_to](https://docs.gtk.org/gtk4/method.Popover.set_pointing_to.html)
uses the coordinate space of the popover's parent, however we used the
active window's coordinate space which was noticeably different when the
tab bar is visible.

Before:
![Screenshot_12-Feb_22-39-07_com mitchellh
ghostty](https://github.com/user-attachments/assets/5263dd9d-f3ab-44f6-baf2-7208d3d3bd01)
After:
![Screenshot_12-Feb_22-41-41_com mitchellh
ghostty-debug](https://github.com/user-attachments/assets/8d3e8bbd-af4e-43a6-a52d-6601a0b66ece)

src/apprt/gtk/Surface.zig

@@ -1270,10 +1270,12 @@ fn showContextMenu(self: *Surface, x: f32, y: f32) void {
         return;
     };
 
+    // Convert surface coordinate into coordinate space of the
+    // context menu's parent
     var point: c.graphene_point_t = .{ .x = x, .y = y };
     if (c.gtk_widget_compute_point(
         self.primaryWidget(),
-        @ptrCast(window.window),
+        c.gtk_widget_get_parent(@ptrCast(window.context_menu)),
         &c.GRAPHENE_POINT_INIT(point.x, point.y),
         @ptrCast(&point),
     ) == 0) {