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Since Emacs draws using X11 primitives, Emacs drawing operations can
erase the GtkDrawingArea used to draw the offscreen widget, which
leads to unpleasant flickering effects, all of which can be prevented
by creating a native X window and drawing to that instead.

* src/xterm.c (x_window_to_frame): Return xwidget window if
appropriate.
(handle_one_xevent): Handle xwidget expose events.

* src/xwidget.c (x_window_to_xvw_map): New variable.
(xwidget_view_from_window): New function.

(xwidget_show_view, xwidget_hide_view)
(offscreen_damage_event): Move to drawing xwidgets with X windows.

(xv_do_draw)
(xwidget_expose): New function.

(xwidget_osr_draw_cb, xwidget_osr_event_forward)
(xwidget_osr_event_set_embedder): Removed.

(xwidget_init_view): Replace use of GtkDrawingArea with that of an X
window.

(x_draw_xwidget_glyph_string): Prevent xwidget views from constantly
generating expose events, and use an X window instead of a
GtkDrawingArea.

(syms_of_xwidget): Initialize x_window_to_xwv_map.

src/xwidget.h (struct xwidget_view): Replace GTK widgets with
X-related fields.

(xwidget_view_from_window, xwidget_expose): New functions.