a11y: Fix bounds check for AT-SPI GetAccessibleAtPoint

In `accessible_at_point`, fix the check whether the
given point is inside of the accessible's bounds.

For that to be the case, the point's x coordinate
must be somewhere between the X position of the
accessible's bounds and that position + width.
(Likewise for the y coordinate and the height.)

the previous check would only work correctly
for children located at a relative location
of (0, 0) within the parent.

With this and the previous commit in place,
the (extended) example from issue #6448 now gives
the expected result:
an accessible object in whose bounds include
requested point (50, 50) lies:

    In [19]: acc.queryComponent().getExtents(pyatspi.WINDOW_COORDS)
    Out[19]: [0, 0, 800, 600]
    In [20]: acc.queryComponent().getAccessibleAtPoint(50, 50, pyatspi.WINDOW_COORDS)
    Out[20]: <Atspi.Accessible object at 0x7fae500e9180 (AtspiAccessible at 0x33455b0)>
    In [21]: acc.queryComponent().getAccessibleAtPoint(50, 50, pyatspi.WINDOW_COORDS).queryComponent().getExtents(pyatspi.WINDOW_COORDS)
    Out[21]: [6, 1, 68, 49]

Fixes: #6448

gtk/a11y/gtkatspicomponent.c

@@ -164,7 +164,7 @@ accessible_at_point (GtkAccessible *parent,
   if (!gtk_accessible_get_bounds (parent, &px, &py, &width, &height))
     return NULL;
 
-  if (!children_only && x >= 0 && x <= width && y >= 0 && y <= height)
+  if (!children_only && x >= px && x <= px + width && y >= py && y <= py + height)
     result = parent;
 
   for (GtkAccessible *child = gtk_accessible_get_first_accessible_child (parent);