Oops. Was introduced when I last changed modeling.
The problem was, for checking that a previous master m is outside the current
influence box of the current master, I was doing "not (lower < m[loc] < upper)".
This is wrong, where lower,peak,upper is the support of previous master.
This fails if lower == peak == m[loc], or m[loc] == peak == upper.
Fixes https://github.com/fonttools/fonttools/issues/1269#issuecomment-397655016
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