(set_buffer_internal): Set windows_or_buffers_changed

(set_buffer_internal): Set windows_or_buffers_changed
only if buffer is displayed somewhere.
(modify_overlay): Use BUF_COMPUTE_UNCHANGED.
(reset_buffer): Set clip_changed to 0 and
prevent_redisplay_optimizations_p to 1.
(Fget_buffer_create): Set prevent_redisplay_optimizations_p to 1.
(modify_overlay): Compute beg/end_unchanged
per buffer.
(Fget_buffer_create): Initialize new members of the buffer
structure.