A DMX address: a universe and a channel in it. Derived from the patch
(see Patch::addresses), which keeps the universe at most
MAX_UNIVERSE and the channel inside the universe.
An instance of a fixture definition. Its address is derived from the
topology, and its position from whichever layout it is looked up in, so
neither is stored here.
Channels addressable in one DMX universe. gled-gpu checks at compile time
that its output buffer holds the same number per universe, so derived
addresses and the DMX packing cannot drift apart.