bug#61580: line-prefix and display text properties cause odd cursor movement

[Darius Foo <darius.foo.tw@gmail.com>]

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When moving the cursor up and down on lines which have both line-prefix and
display text properties, the cursor column isn't preserved, jumping ahead
or back by a fixed amount.

This can be reproduced with the following config

      (add-text-properties 12 13 '(line-prefix "  "))
      (add-text-properties 27 28 '(line-prefix "  "))
      (add-text-properties 57 58 '(line-prefix "    "))
      (add-text-properties 72 73 '(line-prefix "    "))

      (add-text-properties 12 13 '(display "•"))
      (add-text-properties 27 28 '(display "•"))
      (add-text-properties 57 58 '(display "•"))
      (add-text-properties 72 73 '(display "•"))

and this input file (which mimics org-indent).

    * h1
    ** h2
    - a.y.........
    - x...........
    *** h3
    **** h4
    - ....w.......
    - z...........

With the cursor on x, pressing up moves it to y instead of a, and pressing
down moves it back to x. The amount moved depends on the indentation added
by line-prefix: with the cursor on z, pressing up moves it to w.

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