Use Cairo XCB surfaces when XCB is available

Use Cairo XCB surfaces when XCB is available

This lets us get at goodies such as shm support, which aren't
available with the xlib surface.  (bug#52120)

* configure.ac: Test for cairo-xcb if cairo is available on X.
* src/xterm.c (USE_CAIRO_XCB_SURFACE): New define.
(x_begin_cr_clip): Create XCB surfaces if available.
(x_try_cr_xlib_drawable):
(x_scroll_run): Handle XCB surfaces.
(x_term_init): Find XCB visualtype structure.
* src/xterm.h (struct x_display_info): New field `xcb_visual'.
* src/xwidget.c (x_draw_xwidget_glyph_string): Fix integer
overflow.