Cannot set styles for merge-path in set-style

[SimonYde]

          For `merge-path` you have to set it for the new, merged path via `merge-path`'s `stroke:` attribute. You cannot set different stroke or fill styles for different merged parts.

merge-path's stroke cannot be set via set-style (which is a bug I guess).

Originally posted by @johannes-wolf in #57 (comment)

As discussed in a separate issue, it's not possible to define styling for merge-path's in a global scope, unless you define it for every construct using i.e. top-level stroke styling.