Take window shapes into account when processing drag and drop

Take window shapes into account when processing drag and drop

* configure.ac: Test for the Nonrectangular Window Shape
extension.
* msdos/sed1v2.inp: Update.
* src/xterm.c (struct x_client_list_window): New fields for
shapes.
(x_dnd_free_toplevels): Free shapes.
(x_dnd_compute_toplevels): Populate window shapes.
(x_dnd_get_target_window_2): New function.
(x_dnd_get_target_window_1): Test WM state of window before
taking it into account.
(x_dnd_begin_drag_and_drop): Use outer window as the initial
last seen window.
(x_dnd_update_state): Small fixes to frame tracking.
(handle_one_xevent): Handle ShapeNotify events correctly.
(x_term_init): Test for the Nonrectangular Window Shape
extension.
* src/xterm.h (struct x_display_info): New atom `WM_STATE'.