Problem:
When a window is redrawn, `draw_vsep_win`/`draw_hsep_win` paint plain
separator characters (`│`/`─`) along the window's entire edges,
including cells that are connector corners belonging to other windows.
Then `draw_sep_connectors_win` only fixes the corners of that same
window, not connectors in the middle of its edges that belong to
adjacent windows.
If the window that "owns" the connector corner isn't part of the redraw,
the connector is never repainted.
Solution:
Move connector drawing out of the per-window `win_update` and into a
separate pass in `update_screen` that runs after all windows have been
updated.
6657cc3671