mass_on_time.js: switch from Spice.render to Spice.add

I'm not quite happy with how this works from duckpan, but I think that's
a local issue.  Regardless, this alias will be going away, so we need to
make this switch.

Fixes #1073.
Merge pull request #1076 from mwmiller/mot

mass_on_time.js: switch from Spice.render to Spice.add