patch 9.1.0171: Small split-move related improvements

Problem:  small improvements can be made to split-move related
          functions.
Solution: apply them (Sean Dewar):

- Improve some doc comments (frame_flatten should still work for non-current
  tabpages, despite the topframe check, which looks benign, though I'm unsure if
  it's still needed; see #2467).

- f_win_splitmove should check_split_disallowed on wp, not targetwin, as that's
  what win_splitmove checks (though it's probably unnecessary to check
  b_locked_split at all; see #14109, which I hope to get around to finishing at
  some point).

- Make winframe_restore restore window positions for the altframe, which
  winframe_remove changes. This doesn't affect the prior behaviour, as we called
  win_comp_pos after, but as win_comp_pos only works for curtab, and
  winframe_remove supports non-current tabpages, we should undo it. Regardless,
  this should mean we don't need win_comp_pos anymore; adjust tests to check
  that window positions remain unchanged.

  I'm not sure win_comp_pos is needed after last_status anyway if it doesn't
  steal rows from another frame to make room for a new statusline, which
  shouldn't be the case after winframe_remove? To be safe, I'll leave it as is.

closes: #14185

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/evalwindow.c

@@ -1001,8 +1001,8 @@ f_win_splitmove(typval_T *argvars, typval_T *rettv)
 	size = (int)dict_get_number(d, "size");
     }
 
-    // Check if we can split the target before we bother switching windows.
-    if (text_or_buf_locked() || check_split_disallowed(targetwin) == FAIL)
+    // Check if we're allowed to continue before we bother switching windows.
+    if (text_or_buf_locked() || check_split_disallowed(wp) == FAIL)
 	return;
 
     if (curwin != targetwin)