Provide a "flipping through channels" look via animated textures

Title almost says it all :-)

There is one caveat:  Since the TV is 3x2 nodes in size, animation glitches are possible (if somewhat unlikely) if a TV is placed such that it overlaps the boundary between two or more mapblocks.

If that happens, each mapblock will probably display its part of the TV at a different offset in the animation.  I believe there is a minetest.conf setting (for the client) that will force the animations to stay in sync, preventing this issue, but I am led to understand it comes with a performance penalty.