Don't scroll selection on the other screen

Fixes garbage selections when switching to/from the alternate screen.

How to reproduce:

-   Be in primary screen.
-   Select something.
-   Run this (switches to alternate screen, positions the cursor at the
    bottom, triggers selscroll(), and then goes back to primary screen):

        tput smcup; tput cup $(tput lines) 0; echo foo; tput rmcup

-   Notice how the (visual) selection now covers a different line.

The reason is that selscroll() calls selnormalize() and that cannot find
the original range anymore. It's all empty lines now, so it snaps to
"select the whole line".
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Gitea Version: 1.28.0+dev-505-g31b4d79a84