[doc] setup GUI for wsl2

win/wsl.md

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+# GUI
+
+## X11 for Windows
+
+1. download and install [vcxsrc](https://sourceforge.net/projects/vcxsrv/)
+2. double click on XLaunch to launch vcxsrc for Windows
+3. put following setting to `~/.profile.fish`
+```fish
+set -gx DISPLAY (winip):0.0
+# window size scaling factor
+set -gx GDK_SCALE 0.5
+# dpi scaling factor
+set -gx GDK_DPI_SCALE 2
+```
+
+## PulseAudio for Windows
+
+### Windows Part
+1. download and extract [pluseaudio](https://www.freedesktop.org/wiki/Software/PulseAudio/Ports/Windows/Support/) to your preferred location
+2. append following line to `etc/default.pa`:
+```
+load-module module-native-protocol-tcp auth-ip-acl=127.0.0.1;172.16.0.0/12
+```
+3. append following line to `etc/daemon.conf`:
+```
+exit-idle-time = -1
+```
+4. run `bin/pulseaudio.exe` on `cmd/powershell` to start up pulaudio-server for testing
+5. using `nssm` [nssm](https://nssm.cc/download) to make `pulseaudio` run as system service
+
+### WSL Part
+1. install package
+```sh
+sudo apt install libpulse0
+```
+2. setup environment variables on `~/.profile.fish`:
+```fish
+set -gx PULSE_SERVER tcp:(winip)
+```
+3. play sound using `ffplay` to test