Bug 1720497 - Do not require MOZ_ENABLE_WAYLAND if Xwayland is not available, r=stransky

If `WAYLAND_DISPLAY` is set but `DISPLAY` is not, we have every reason
to assume that we are in a Wayland-only session. Enable the Wayland
backend unconditionally in this case.

Note: We do not bother to further sanity check the content of the env
variables as these can be considered client setup bugs.

Differential Revision: https://phabricator.services.mozilla.com/D120216

toolkit/xre/nsAppRunner.cpp

@@ -4081,9 +4081,10 @@ bool IsWaylandEnabled() {
     return false;
   }
 
+  const char* x11Display = PR_GetEnv("DISPLAY");
   // MOZ_ENABLE_WAYLAND is our primary Wayland on/off switch.
   const char* waylandPref = PR_GetEnv("MOZ_ENABLE_WAYLAND");
-  bool enableWayland = (waylandPref && *waylandPref);
+  bool enableWayland = !x11Display || (waylandPref && *waylandPref);
   if (!enableWayland) {
     const char* backendPref = PR_GetEnv("GDK_BACKEND");
     enableWayland = (backendPref && strncmp(backendPref, "wayland", 7) == 0);