remove some dead code
ignore buffer.raw output
share the scene among all worker threads

Figuring out how to do this was one of the hardest things I encountered
in porting Luculentus. Thanks huon on #rust, this is how to do it.
do not derive Show

That was done for debugging purposes only, it is no longer necessary.
remove resolved TODO
remove TODO

It is not resolved, and the method _can_ probably be optimised by
transmuting the memory to `[u8]`. However, it is not a bottleneck, so
there is no need to optimise it.
put canvas size in one place