Optimize away duplicate conversion nodes.

Sometimes an effect can be used by two other effects that both demand
the same conversion. If so, we should simply insert a conversion after
that effect and connect _all_ outputs to that conversion, since
conversions to linear/sRGB/premultiplied never hurt for us.

Add unit tests for the gamma and colorspace cases. I could have added
for the alpha case, too, but it got very tedious. :-)