Make logging cost free when there is no output target (#12247)

The logging streams now do almost no work when there is no output target for them.

For example, if LL_VERBOSE has no output targets, then `verbosestream << x` will return a StreamProxy with a null target. Any further `<<` operations applied to it will do nothing.

src/util/stream.h

@@ -0,0 +1,70 @@
+/*
+Minetest
+Copyright (C) 2022 Minetest Authors
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program 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 Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include <iostream>
+#include <string>
+#include <functional>
+
+template<int BufferLength, typename Emitter = std::function<void(const std::string &)> >
+class StringStreamBuffer : public std::streambuf {
+public:
+	StringStreamBuffer(Emitter emitter) : m_emitter(emitter) {
+		buffer_index = 0;
+	}
+
+	int overflow(int c) {
+		push_back(c);
+		return c;
+	}
+
+	void push_back(char c) {
+		if (c == '\n' || c == '\r') {
+			if (buffer_index)
+				m_emitter(std::string(buffer, buffer_index));
+			buffer_index = 0;
+		} else {
+			buffer[buffer_index++] = c;
+			if (buffer_index >= BufferLength) {
+				m_emitter(std::string(buffer, buffer_index));
+				buffer_index = 0;
+			}
+		}
+	}
+
+	std::streamsize xsputn(const char *s, std::streamsize n) {
+		for (int i = 0; i < n; ++i)
+			push_back(s[i]);
+		return n;
+	}
+private:
+	Emitter m_emitter;
+	char buffer[BufferLength];
+	int buffer_index;
+};
+
+class DummyStreamBuffer : public std::streambuf {
+	int overflow(int c) {
+		return c;
+	}
+	std::streamsize xsputn(const char *s, std::streamsize n) {
+		return n;
+	}
+};