Fix frame synchronization with scroll bar movement

Fix frame synchronization with scroll bar movement

* src/xfns.c (x_set_inhibit_double_buffering): Stop condeming
scroll bars.

* src/xterm.c (x_scroll_bar_create): Create an InputOnly window.
Update event masks accordingly and stop allocating back buffer.
(x_scroll_bar_remove): Stop deallocating back buffer.
(XTset_vertical_scroll_bar, x_scroll_bar_set_handle): Draw onto
the edit window so they can be synchronized with buffer flips.
(x_scroll_bar_clear): Redraw scroll bars instead of just
clearing them.
(x_scroll_bar_handle_expose, x_scroll_bar_redraw): New
functions.
(x_scroll_bar_expose, x_scroll_bar_end_update): Delete
functions.
(handle_one_xevent): Update exposure logic accordingly.

* src/xterm.h (struct scroll_bar): Remove `x_drawable' field.