Fallback wayland cursor theme to XCURSOR_THEME

If the wayland environment doesn't set the cursor properly but the
XCURSOR_THEME variable is set, it should fallback to it.

source/wayland/display.c

@@ -1210,8 +1210,16 @@ static gboolean wayland_cursor_reload_theme(guint scale) {
   }
   cursor_size *= scale;
 
+  char *cursor_name = wayland->cursor.theme_name;
+  if(cursor_name == NULL) {
+    char *env_cursor_theme = (char *)g_getenv("XCURSOR_THEME");
+    if(strlen(env_cursor_theme) > 0) {
+      cursor_name = env_cursor_theme;
+    }
+  }
+
   wayland->cursor.theme =
-      wl_cursor_theme_load(wayland->cursor.theme_name, cursor_size, wayland->shm);
+      wl_cursor_theme_load(cursor_name, cursor_size, wayland->shm);
   if (wayland->cursor.theme != NULL) {
     const char *const *cname = (const char *const *)wayland->cursor.name;
     for (cname = (cname != NULL) ? cname : wayland_cursor_names;