Optimize away duplicate conversion nodes.

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. :-)