Fix alternate-screen scrolling

The alternate-screen mode disables scrollback. This
means that we delete lines off of the top of the
screen when scrolling, without saving them.

But we weren't keeping a count of those lines, which
meant a stable index was lost; leaving the mux-server
unable to properly track scrolling.

Now we keep track of the stable_row_index_offset, even
when there is no scrollback.  This turns out to be the
exact same logic as when scrollback is enabled, but
after "erase_scrollback" has been called. In both cases,
stable_row_index_offset is used to count the number
of scrollback-lines, that don't exist in the buffer.

As per issue #6166

term/src/screen.rs

@@ -724,7 +724,7 @@ impl Screen {
             self.lines.remove(remove_idx);
         }
 
-        if remove_idx == 0 && self.allow_scrollback {
+        if remove_idx == 0 {
             self.stable_row_index_offset += lines_removed;
         }