#1570: on macos, use the cairo backend for transparent windows

git-svn-id: https://xpra.org/svn/Xpra/trunk@18900 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/client/gtk_base/gtk_client_base.py

@@ -1061,7 +1061,7 @@ def get_client_window_classes(self, w, h, metadata, override_redirect):
         ms = min(self.sx(self.gl_texture_size_limit), *self.gl_max_viewport_dims)
         #win32 opengl doesn't do alpha (not sure why):
         alpha = metadata.boolget("has-alpha", False)
-        if self.GLClientWindowClass is None or not self.opengl_enabled or w>ms or h>ms or (WIN32 and (alpha or override_redirect)):
+        if self.GLClientWindowClass is None or not self.opengl_enabled or w>ms or h>ms or (WIN32 and (alpha or override_redirect)) or (OSX and alpha):
             return [self.ClientWindowClass]
         return [self.GLClientWindowClass, self.ClientWindowClass]