Clean up X11 double buffering code

Clean up X11 double buffering code

This fixes several latent bugs where code went down the path
with double buffering enabled when it wasn't, and vice versa.

* src/xfns.c (x_set_inhibit_double_buffering): Improve
commentary and only define when HAVE_XDBE.
(x_mark_frame_dirty): Only set buffer flip flag when HAVE_XDBE.
(initial_set_up_x_back_buffer): Clean up coding style and remove
unnecessary block_input pair.
(Fx_double_buffered_p): Always return nil if !HAVE_XDBE.
(x_frame_parm_handlers): Don't set double buffering handler if
!HAVE_XDBE.

* src/xftfont.c (xftfont_drop_xrender_surfaces, xftfont_driver):
Only define when XDBE is available.

* src/xterm.c (x_drop_xrender_surfaces): Likewise.
(x_clear_window): Don't test double buffering flags when
!HAVE_XDBE.
(show_back_buffer): Only define when HAVE_XDBE.
(x_flip_and_flush): Don't try to flip when !HAVE_XDBE.
(XTframe_up_to_date): Likewise.
(XTbuffer_flipping_unblocked_hook): Only define when Xdbe is
available.
(x_clear_area): Don't test double buffering flags when Xdbe is
not available.
(flush_dirty_back_buffer_on): Don't define if there's no DBE.
(handle_one_xevent, x_create_terminal): Likewise.

* src/xterm.h (FRAME_X_DRAWABLE): Fix coding style.