try hard to preserve the vertical scrolling position when auto-scrolling dot

* There are cases, especially where the entire buffer is piped through some
  external process or when removing and reinserting large parts of the buffer,
  that dot changes very little, but the vertical scrolling position gets resets.
  This is especially noticable with the macro @^U{[: HECcat$ ]:},
  but also with M#cf (clang-format wrapper from "Useful macros").
* We now try to preserve the vertical position ("first visible line")
  before scrolling caret.

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