Misleading naming conventions regarding channel ordering

[stohrendorf]

The encoding expects a channel ordering of BGRA, whereas every function argument or function name uses RGBA. Even more confusing, the decoding returns RGBA, and not BGRA, which means that the semantics of the data suddenly changes when doing a simple encode/decode roundtrip (given you could put in in-memory data as mentioned in #33), as the channel ordering gets changed.