MeshUpdateQueue: Add a MapBlock cache that minimizes the amount of MapBlock copying done in the main thread

Cache size is configurable by the meshgen_block_cache_size (default 20 MB).

New profiler stats:
- MeshUpdateQueue MapBlock cache hit %
- MeshUpdateQueue MapBlock cache size kB

Removes one type of stutter that was seen on the client when received MapBlocks
were being handled. (the "MeshMakeData::fill" stutter)

Kind of related to at least #5239

Originally preceded by these commits, now includes them:
- Move the mesh generator thread into src/mesh_generator_thread.{cpp,h}
- mesh_generator_thread.cpp: Update code style
- MeshUpdateThread: Modify interface to house a different implementation: Actual functionality will be changed by next commits.
- MeshMakeData: Add fillBlockData() interface (so that caller can fill in stuff from eg. a MapBlock cache)

src/mesh_generator_thread.h

@@ -0,0 +1,135 @@
+/*
+Minetest
+Copyright (C) 2013, 2017 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+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.
+*/
+
+#ifndef MESH_GENERATOR_THREAD_HEADER
+#define MESH_GENERATOR_THREAD_HEADER
+
+#include "mapblock_mesh.h"
+#include "threading/mutex_auto_lock.h"
+#include "util/thread.h"
+
+struct CachedMapBlockData
+{
+	v3s16 p;
+	MapNode *data; // A copy of the MapBlock's data member
+	int refcount_from_queue;
+	int last_used_timestamp;
+
+	CachedMapBlockData();
+	~CachedMapBlockData();
+};
+
+struct QueuedMeshUpdate
+{
+	v3s16 p;
+	bool ack_block_to_server;
+	bool urgent;
+	int crack_level;
+	v3s16 crack_pos;
+	MeshMakeData *data; // This is generated in MeshUpdateQueue::pop()
+
+	QueuedMeshUpdate();
+	~QueuedMeshUpdate();
+};
+
+/*
+	A thread-safe queue of mesh update tasks and a cache of MapBlock data
+*/
+class MeshUpdateQueue
+{
+	enum UpdateMode {
+		FORCE_UPDATE,
+		SKIP_UPDATE_IF_ALREADY_CACHED,
+	};
+public:
+	MeshUpdateQueue(Client *client);
+
+	~MeshUpdateQueue();
+
+	// Caches the block at p and its neighbors (if needed) and queues a mesh
+	// update for the block at p
+	void addBlock(Map *map, v3s16 p, bool ack_block_to_server, bool urgent);
+
+	// Returned pointer must be deleted
+	// Returns NULL if queue is empty
+	QueuedMeshUpdate * pop();
+
+	u32 size()
+	{
+		MutexAutoLock lock(m_mutex);
+		return m_queue.size();
+	}
+
+private:
+	Client *m_client;
+	std::vector<QueuedMeshUpdate*> m_queue;
+	std::set<v3s16> m_urgents;
+	std::map<v3s16, CachedMapBlockData*> m_cache;
+	Mutex m_mutex;
+
+	// TODO: Add callback to update these when g_settings changes
+	bool m_cache_enable_shaders;
+	bool m_cache_use_tangent_vertices;
+	bool m_cache_smooth_lighting;
+	int m_meshgen_block_cache_size;
+
+	CachedMapBlockData* cacheBlock(Map *map, v3s16 p, UpdateMode mode,
+			size_t *cache_hit_counter=NULL);
+	CachedMapBlockData* getCachedBlock(const v3s16 &p);
+	void fillDataFromMapBlockCache(QueuedMeshUpdate *q);
+	void cleanupCache();
+};
+
+struct MeshUpdateResult
+{
+	v3s16 p;
+	MapBlockMesh *mesh;
+	bool ack_block_to_server;
+
+	MeshUpdateResult():
+		p(-1338,-1338,-1338),
+		mesh(NULL),
+		ack_block_to_server(false)
+	{
+	}
+};
+
+class MeshUpdateThread : public UpdateThread
+{
+public:
+	MeshUpdateThread(Client *client);
+
+	// Caches the block at p and its neighbors (if needed) and queues a mesh
+	// update for the block at p
+	void updateBlock(Map *map, v3s16 p, bool ack_block_to_server, bool urgent);
+
+	v3s16 m_camera_offset;
+	MutexedQueue<MeshUpdateResult> m_queue_out;
+
+private:
+	MeshUpdateQueue m_queue_in;
+
+	// TODO: Add callback to update these when g_settings changes
+	int m_generation_interval;
+
+protected:
+	virtual void doUpdate();
+};
+
+#endif