patch 9.1.1079: GUI late startup leads to uninitialized scrollbars

patch 9.1.1079: GUI late startup leads to uninitialized scrollbars

Problem:  GUI late startup leads to uninitialized scrollbars
Solution: initialize scrollbars for all windows in all tabpages
          (Yee Cheng Chin)

GUI startup was erroneously only initializing scrollbars for all windows
in current tab, instead of all tabs. This breaks if the user has created
tab pages before using `:gui` command to enter GUI mode, or sourced a
session file in vimrc.

closes: #16588
related: macvim-dev/macvim#862

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>