Show only single instance of mathjax-helper exists

Open up at least two markdown previews and Dev Tools. It is instructed
to .log the number of times LaTex rendering has been toggled for a
particular preview, however the number only ever increases even when you
switch to a different preview. I made a mistake in my consideration of
the scope of renderer, only one instance is ever created which serves
all markdownPreviewView's and hence only one mathjax-helper serves all
aswell.
Fix per preview toggle of LaTex rendering