Reduce complexity of scroll bar window protection code

Reduce complexity of scroll bar window protection code

It turns out my previous theories of why ClientMessages were
generated were wrong, and they are just generated so we can set
finish to X_EVENT_GOTO_OUT to have them delivered immediately.

This allows for the code to be simplified greatly, by not
unpacking pointers to the raw window into the client messages.

* src/xterm.c (x_unprotect_window_for_callback): Return the
window removed from the queue, or nil if none.
(x_scroll_bar_to_input_event)
(x_horizontal_scroll_bar_to_input_event): Use window provided by
x_unprotect_window_for_callback.
(handle_one_xevent): Pass dpyinfo to functions that need it.
(x_free_frame_resources): Remove "first scroll bar request"
code.

* src/xterm.h (struct x_display_info): Likewise.