TeX input: improve heuristics for invisible apply

From the discussion at https://github.com/mathjax/MathJax/issues/1305#issuecomment-158961615

The TeX input jax does put invisible times following things like \sin and \dim, but only in certain circumstances, and (unfortunately) that doesn't currently include when \dim is followed by another operator. The heuristic for adding invisible times could be altered to include more situations, and probably should.