"Artifact" when plotting labels with `outline=True`

[clwgg]

I've come across a weird "artifact" that can be triggered when plotting labels with outline=True:

sdata.pl.render_labels(outline=False).pl.show()

sdata.pl.render_labels(outline=True).pl.show()

It's a little hard to see, but in the 2nd figure, there appears to be some kind of "shade" in the top-left corner of the plot.

I went digging a bit for possible causes -- please see the PR linked below for a potential fix.