The data returned by `Pattern.getRenderProps()` was not serializable as
we were returning `this` all over the place, thereby including marcors,
log methods, cyclic object references, and so on.
This commit changes that by implementing a `.asRenderProp()` method on
all of the various objects (stack, part, path, point, snippet,
attributes, svg) and only including data that can be serialized.
In addition, we no longer include the logs in the renderProps because
they are not related to rendering the pattern.
Instead, the new method `Pattern.getLogs()` gives you the logs.
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