os_window_size within a session only affects new windows.

[salmankhilji]

Describe the bug
os_window_size within a session only affects new OS windows. (It is not clear whether this is by design or not.)

To Reproduce
Steps to reproduce the behavior:

1. Launch kitty like this:

kitty --config NONE -o remember_window_size=no --session <( cat <<EOF
os_window_size 80c 25c

new_tab A1
launch

new_tab B1
launch

os_window_size 80c 25c
new_os_window
os_window_size 80c 25c

new_tab A2
launch

new_tab B2
launch
EOF
)

Please note that we pass the remember_window_size=no to avoid kitty loading the cached window size from the .cache directory.

Screenshots

We get one small and one large window. I would expect the os_window_size to also size the window. (We tried not once but twice within the session file above.)

Environment details

kitty 0.25.0 (afebea8635) created by Kovid Goyal
Linux byteevo 5.13.0-39-generic #44~20.04.1-Ubuntu SMP Thu Mar 24 16:43:35 UTC 2022 x86_64
Ubuntu 20.04.4 LTS byteevo /dev/tty

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.4 LTS"
Running under: X11
Frozen: False
Paths:
  kitty: /home/precor/Developer/kitty/kitty/launcher/kitty
  base dir: /home/precor/Developer/kitty
  extensions dir: /home/precor/Developer/kitty/kitty
  system shell: /bin/bash
Loaded config overrides:
  remember_window_size no

Config options different from defaults:
remember_window_size False

Important environment variables seen by the kitty process:
	PATH                                /home/precor/.nvm/versions/node/v14.18.2/bin:/opt/qt/5.15.2/gcc_64/bin:/home/precor/.local/flutter/bin:/home/precor/.pub-cache/bin:/home/precor/Android/Sdk/build-tools/30.0.3:/home/precor/Android/Sdk/emulator:/home/precor/Android/Sdk/platform-tools:/home/precor/Android/Sdk/cmdline-tools/latest/bin:/home/precor/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
	LANG                                en_US.UTF-8
	SHELL                               /bin/bash
	DISPLAY                             :0
	USER                                precor
	XDG_CONFIG_DIRS                     /etc/xdg/xdg-ubuntu:/etc/xdg
	XDG_MENU_PREFIX                     gnome-
	XDG_SESSION_DESKTOP                 ubuntu
	XDG_SESSION_TYPE                    x11
	XDG_CURRENT_DESKTOP                 ubuntu:GNOME
	XDG_SESSION_CLASS                   user
	XDG_RUNTIME_DIR                     /run/user/1000
	XDG_DATA_DIRS                       /usr/share/ubuntu:/usr/local/share/:/usr/share/

Additional context
kitty is build from source, master branch @ afebea86.