where fontc' lack of rounding leads to deltas that are off-by-one compared with fontmake/fonttools in Gelasio's MVAR table

#1043 (comment)

Fixes #1043 and #235

This commit also adds a test from fonttools Tests/varLib/models_test.py that demostrates how our previous approach of late-rounding of deltas can actually lead to an excessive accummulation of rounding errors when the deltas are actually used for interpolation.

they were indeed useless... thanks Colin