Package macOS releases as a universal binary.

* Minimum macOS version is raised to 10.11.
* App bundles ship 3 versions of the runtime and engine binaries,
  and a fat launcher that selects the appropriate runtime/apphost.
* Mono is used for macOS 10.11 - 10.14, or if OPENRA_PREFER_MONO
  environment variable has been set.

packaging/macos/apphost-mono.c

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2007-2022 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+
+//
+// A custom apphost is required (instead of just invoking `mono OpenRA.dll ...` directly)
+// because macOS will only properly associate dock icons and tooltips to windows that are
+// created by a process in the Contents/MacOS directory (not subdirectories).
+//
+// Based on https://github.com/mono/monodevelop/blob/main/main/build/MacOSX/monostub.mm
+
+#include <dlfcn.h>
+#include <stdio.h>
+#include <sys/resource.h>
+
+typedef int (* mono_main)(int argc, char **argv);
+
+int main(int argc, char **argv)
+{
+	// TODO: This snippet increasing the open file limit was copied from
+	// the monodevelop launcher stub. It may not be needed for OpenRA.
+	struct rlimit limit;
+	if (getrlimit(RLIMIT_NOFILE, &limit) == 0 && limit.rlim_cur < 1024)
+	{
+		limit.rlim_cur = limit.rlim_max < 1024 ? limit.rlim_max : 1024;
+		setrlimit(RLIMIT_NOFILE, &limit);
+	}
+
+	void *libmono = dlopen(argv[1], RTLD_LAZY);
+	if (libmono == NULL)
+	{
+		fprintf(stderr, "Failed to load libmonosgen-2.0.dylib: %s\n", dlerror());
+		return 1;
+	}
+
+	mono_main _mono_main = (mono_main)dlsym(libmono, "mono_main");
+	if (!_mono_main)
+	{
+		fprintf(stderr, "Could not load mono_main(): %s\n", dlerror());
+		return 1;
+	}
+
+	return _mono_main(argc - 1, &argv[1]);
+}