Add meshnode drawtype.

doc/lua_api.txt

src/client.cpp

src/content_mapblock.cpp

src/mapblock_mesh.cpp

src/mapblock_mesh.h

src/mesh.cpp

src/mesh.h

src/nodedef.cpp

src/nodedef.h

src/script/common/c_content.cpp

src/script/cpp_api/s_node.cpp