Wayland: Detect SSD by querying compositor

Much more reliable than checking environment variables. Does
add ~1ms to startup time on Wayland.

kitty/constants.py

@@ -137,16 +137,29 @@ def detect_if_wayland_ok():
         return False
     if 'KITTY_DISABLE_WAYLAND' in os.environ:
         return False
-    if not os.path.exists(glfw_path('wayland')):
+    wayland = glfw_path('wayland')
+    if not os.path.exists(wayland):
         return False
-    cd = os.environ.get('XDG_CURRENT_DESKTOP')
-    if cd:
-        cd = frozenset(cd.split(':'))
-        if 'GNOME' in cd:
-            # GNOME does not support xdg-decorations
-            # https://gitlab.gnome.org/GNOME/mutter/issues/217
-            return False
-    return True
+    # GNOME does not support xdg-decorations
+    # https://gitlab.gnome.org/GNOME/mutter/issues/217
+    import ctypes
+    lib = ctypes.CDLL(wayland)
+    check = lib.glfwWaylandCheckForServerSideDecorations
+    check.restype = ctypes.c_char_p
+    check.argtypes = ()
+    try:
+        ans = bytes(check())
+    except Exception:
+        return False
+    if ans == b'NO':
+        print(
+                'Your Wayland compositor does not support server side window decorations,'
+                ' disabling Wayland. You can force Wayland support using the'
+                ' linux_display_server option in kitty.conf'
+                ' See https://drewdevault.com/2018/01/27/Sway-and-client-side-decorations.html'
+                ' for more information.',
+                file=sys.stderr)
+    return ans == b'YES'
 
 
 def is_wayland(opts=None):