Closes#3634
To produce inferred deltas that will be correct given OpenType's gvar
semantics, fontTool's IUP optimisation module checks the equality of
some points. However, this happens before the points are rounded,
whereas the point comparison that happens at runtime will occur after
the points are rounded (as is necessary to serialise glyf), which leads
to diverging semantics and so diverging and incorrect implied deltas.
This leads to significant visual artefacts, e.g. where large deltas that
should be inferred based on previous values are instead interpreted as 0
at runtime.
I suspect this has gone undetected as the subsetter normally works with
rounded points; in the rarer case that partial VF instancing is
occurring with a different default position, however, varLib.instancer
will calculate and apply the relevant deltas to the font's original
coordinates to effect the new default position, which leads to unrounded
points in memory. This commit ensures that we round directly before
optimising (but still after calculating `glyf` metrics, for backward
compatibility).
07351d12e6