wayland: add SDL environment variable.

Author: mahiuchun

src/config/graphical-session/wayland.md

@@ -52,6 +52,8 @@ applications also require installing the `kwayland` package.
 [EFL](https://wayland.freedesktop.org/efl.html)-based applications require
 setting the environment variable `ELM_DISPLAY=wl`, and can have issues without
 it, due to not supporting XWayland properly.
+[SDL](https://libsdl.org)-based applications require setting the environment
+variable `SDL_VIDEODRIVER=wayland`.
 [GTK+](https://wiki.gnome.org/Initiatives/Wayland/GTK%2B)-based applications
 should use the Wayland backend automatically. Information about other toolkits
 can be found in the [Wayland