Minor clientmap improvements.

- Avoid calculating isBlockInSight for blocks without meshes.
- Add metric for how many blocks the client has currently loaded.
- Make some variables constant.

src/mapsector.h

@@ -62,6 +62,7 @@ public:
 
 	bool empty() const { return m_blocks.empty(); }
 
+	int size() const { return m_blocks.size(); }
 protected:
 
 	// The pile of MapBlocks