Fix windows SEH hook when crashing in an std::thread

Follows 107d3d461f and other 'pthread->std::thread' diffs.

Windows uses Structured Exception Handling to allow reporting errors
(both C++ and hardware) nicely. This works by wrapping the code in a
__try __catch block.
The pthread wrapper did this automatically, but we now need to do it
explicitly for std::thread.

Tested by: Stan
Differential Revision: https://code.wildfiregames.com/D3261
This was SVN commit r24530.

source/ps/Threading.cpp

@@ -0,0 +1,41 @@
+/* Copyright (C) 2021 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "precompiled.h"
+
+#include "Threading.h"
+
+#include <thread>
+
+static bool g_MainThreadSet;
+static std::thread::id g_MainThread;
+
+bool Threading::IsMainThread()
+{
+	// If SetMainThread hasn't been called yet, this is probably being
+	// called at static initialisation time, so it must be the main thread
+	if (!g_MainThreadSet)
+		return true;
+
+	return g_MainThread == std::this_thread::get_id();
+}
+
+void Threading::SetMainThread()
+{
+	g_MainThread = std::this_thread::get_id();
+	g_MainThreadSet = true;
+}