patch 9.1.0118: Use different restoration strategy in win_splitmove

patch 9.1.0118: Use different restoration strategy in win_splitmove

Problem:  saving and restoring all frames to split-move is overkill now
          that WinNewPre is not fired when split-moving.
Solution: defer the flattening of frames until win_split_ins begins
          reorganising them, and attempt to restore the layout by
          undoing our changes. (Sean Dewar)

This also means we no longer must allocate.

related: #14042

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