This adds strictness to the 'Envelope' function which is the second param. to 'TotalOffset'.  Specifically, it adds strictness under the `Option` by grabbing the envelope function and forcing it to be evaluated (the '\f -> f `seq` ()' lambda).

The reason for this is that building a fingertree (to make a Path) creates a giant chain of thunks to calculate the total envelope.  We don't lose anything from strictly evaluating, because the envelope will always be used eventually.  There's still quite a bit of laziness under that 'f' (which would probably be highly beneficial to remove), but I'm not sure how to do so yet.  I have an idea, and if it works out, this can go back to something much cleaner.