Node texture animation

doc/lua_api.txt

games/minimal/mods/default/init.lua

...l/mods/default/textures/default_lava_flowing_animated.png

...al/mods/default/textures/default_lava_source_animated.png

src/clientserver.h

src/content_mapblock.cpp

src/game.cpp

src/itemdef.cpp

src/mapblock_mesh.cpp

src/mapblock_mesh.h

src/nodedef.cpp

src/nodedef.h

src/scriptapi.cpp

src/test.cpp

src/tile.cpp

src/tile.h