Implement F12 game reset (MRI only)

Can be disabled with "enableReset=false".

While at it, also replace the flakey volatile bool flags
with proper atomics.

src/sharedstate.cpp

@@ -330,11 +330,20 @@ void SharedState::checkShutdown()
 	if (!p->rtData.rqTerm)
 		return;
 
-	p->rtData.rqTermAck = true;
+	p->rtData.rqTermAck.set();
 	p->texPool.disable();
 	scriptBinding->terminate();
 }
 
+void SharedState::checkReset()
+{
+	if (!p->rtData.rqReset)
+		return;
+
+	p->rtData.rqReset.clear();
+	scriptBinding->reset();
+}
+
 Font &SharedState::defaultFont() const
 {
 	return *p->defaultFont;