patch 9.1.0175: wrong window positions with 'winfix{width,height}'

patch 9.1.0175: wrong window positions with 'winfix{width,height}'

Problem:  winframe functions incorrectly recompute window positions if
          the altframe wasn't adjacent to the closed frame, which is
          possible if adjacent windows had 'winfix{width,height}' set.

Solution: recompute for windows within the parent of the altframe and
          closed frame. Skip this (as before) if the altframe was
          top/left, but only if adjacent to the closed frame, as
          positions won't change in that case. Also correct the return
          value documentation for win_screenpos. (Sean Dewar)

The issue revealed itself after removing the win_comp_pos call below
winframe_restore in win_splitmove. Similarly, wrong positions could result from
windows closed in other tabpages, as win_free_mem uses winframe_remove (at least
until it is entered later, where enter_tabpage calls win_comp_pos).

NOTE: As win_comp_pos handles only curtab, it's possible via other means for
positions in non-current tabpages to be wrong (e.g: after changing 'laststatus',
'showtabline', etc.). Given enter_tabpage recomputes it, maybe it's intentional
as an optimization? Should probably be documented in win_screenpos then, but I
won't address that here.

closes: #14191

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