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Its effective range needs to be clamped between 0 and (window height /
4), so it's better to have this constraint in a single place.

* src/window.c (window_scroll_margin): New function.
(window_scroll_pixel_based, window_scroll_line_based):
(Frecenter, Fmove_to_window_line):
* src/xdisp.c (try_scrolling, try_cursor_movement):
(redisplay_window, try_window, try_window_id): Use it.