varLib: use vmtx to compute phantom pts; fix sign of bottomSideY
varLib: use vmtx to compute phantom pts; fix sign of bottomSideY

when 'vmtx' is present in a font, use that to compute the third and fourth 'phantom points'.
When not present, we use the glyph bbox yMax and yMin coords.

NOTE that previously the bottomSideY was incorrectly set to -glyph.yMin (with a minus sign).
However, the minus is not needed when we use the bbox.
Positive vertical advance grows towards negative Y axis.

fonttools/fonttools,googlefonts/fonttools