Reduce non-toolkit scroll bar flicker

Reduce non-toolkit scroll bar flicker

* src/xterm.c (x_scroll_bar_create): Initialize double buffer
for scroll bar.
(x_scroll_bar_set_handle)
(x_scroll_bar_remove, x_scroll_bar_expose): Draw to back buffer
instead.
(x_scroll_bar_end_update): New function.
(x_scroll_bar_clear): Fill the scroll bar with the background
instead when double buffered.
* src/xterm.h (struct scroll_bar): New field `x_drawable'.