add softmax test with more extra padding

[AaronDavidNewman]

Show effects of softmax with larger amounts of whitespace in a measure. See discussion in PR 1376 which was cancelled and replaced with this one.

@rvilarl wrote in previous PR:

shouldn't we change the default from 100 to 5 as well?

That would change all the test cases. Do we want to do that?
I'm also open to making it easier to set the global softmax. It seems like it is defined in 2 places. It seemse like it should be defined in one place, either be in tables, or in the font metrics.

I would go for that, @0xfe what do you think?

[0xfe]

Tests look good. Also, single place for global softmax sounds good, so long as we can still do local overrides.