emacs pgtk doesn't set WAYLAND-DISPLAY

[Julien Cubizolles]

In Wayland, the following environment variables are set in a shell

WAYLAND_DISPLAY=wayland-0
DISPLAY=:0

In Emacs built with pgtk, only DISPLAY is set, with
DISPLAY=wayland-0.

I don't know if there is a good reason for that but it causes the
matplotlib python program to fail as it seems to expect a "valid" value
for DISPLAY or WAYLAND_DISPLAY to be set (see
https://github.com/matplotlib/matplotlib/issues/18377).

Maybe Emacs could just copy both the WAYLAND_DISPLAY and DISPLAY
environment variables instead of merging both ?

-- 
Julien Cubizolles