Improve ctx with fixed alpha channel

[npentrel]

I fixed the rgba method to have values from 0-1.0 for alpha channel (as I think is customary) in the ctx library and added the error handling that there was a todo for.

[MatthewWilkes]

This looks good, but I'm not convinced that the comment here is correct, trying on an actual badge I'm having trouble getting correct behaviour for the 0..256 range for r,g,b

[npentrel]

I have only tested with the simulator where this does seem to work for me. What behaviour are you seeing?

[MatthewWilkes]

I meant the todo in the simulator code. It talks about automatically dispatching depending on the 0..255 or 0..1 ranges, if I'm understanding correctly. However, on the physical badge I was seeing only the 0..1 range working right. Let me check this again.

[npentrel]

Is that in _sim.py? I only see a TODO there about LEDs which is unrelated I think

[MatthewWilkes]

I'd meant the todo you referenced, but I double checked and it's working fine. I'm not sure what had confused me before, sorry for the delay merging.