patch 9.1.1782: buffer-listener callbacks may not match buffer content

patch 9.1.1782: buffer-listener callbacks may not match buffer content

Problem:  buffer-listener callbacks may not match buffer content, since
          they are buffered until the screen is updated.
Solution: Allow to handle buffer-callbacks un-buffered, meaning to
          handle those changes as soon as they happen (Paul Ollis).

fixes: #18183
closes: #18295

Signed-off-by: Paul Ollis <paul@cleversheep.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>