Add preparations for animation frame timing support

Add preparations for animation frame timing support

* src/xterm.c (x_sync_get_monotonic_time)
(x_sync_current_monotonic_time, x_sync_note_frame_times): New
functions.
(x_sync_wait_for_frame_drawn_event, x_sync_update_begin)
(x_sync_handle_frame_drawn): Note frame times.
(x_display_set_last_user_time): Check if the X server time is
probably the same as CLOCK_MONOTONIC.

* src/xterm.h (struct x_display_info, struct x_output): New
fields and flags for clock handling and frame times.