Make SU (Scroll Up) add lines to scrollback
Make SU (Pan Down/Scroll Up) add lines to the scrollback buffer when there are no margins or when the top margin is line 1. This is the behavior of xterm, urxvt, and wezterm. mlterm differs by also adding lines when the top margin is not line 1. VTE never adds lines to the scrollback when invoking SU.
It's also consistent with the behavior for newline and index (long ago, konsole only added lines to the scrollback when there were no margins, but BUG #37890 requested to change that behavior for the sake of tinyfugue, a MUD client with a non-scrolling portion on the bottom of the screen, see commit 29a94111).
We were already adding a single line to the scrollback buffer, since scrollUp() is called by IND (ESC D) and '\n' (a61f5d98, which introduced SU and SD, moved adding a line to scrollback from index() to scrollUp() verbatim).