There is no longer a requirement that all the masters have exactly the same base color glyphs as the default masters. Similarly, it's no longer required that all masters' LayerLists have the same total count of layers. It is sufficient that, for a base color glyph in the default master, a non-default master may (or may not) contain one with the same name and same effective number of layers (which in turn can be laid out differently in the respective LayerLists).
This provides greater flexibility when working with variable font project with sparse glyph sets.
previously we only reused the VarIndexBase of a previously seen variable table when the current's varIdxes were _fully_ equal to one of the previous; now we also try to find a match anywhere in the accummulated list of self.varIdxes, including a partial match at the tail of the list.
When multiple variable tables refer to the same delta-sets they can now share the same VarIndexBase so the resulting DeltaSetIndexMap is a bit smaller.
For simplicity, we only reuse VarIndexBase when variable tables fully share (ie. same, and same number of) varIdxes; potentially we could reuse subsets of varIdxes (e.g. a VarColoStop.Alpha has a +0.5 delta, and later on elsewhere a PaintVarSolid.Alpha has a similar +0.5 delta; the latter could have a VarIndexBase that reuses an existing DeltaSetIndexMap entry for the former), but for now this I think is good enough.
Before we were too greedy in the way we converted subtables to VarType. E.g. If a PaintTransform wrapping a PaintRadialGradient contained variations in the Affine2x3, we would incorrectly convert also the gradient's ColorLine (and ColorStops) to VarColorLine, VarColorStop, etc. (even if the gradient was not variable!). Instead we want skip traversing a given subtable including its children when the predicate doesn't match.
Introduce three compilation modes:
1. Pure python: only uses the existing fonttools packing and overflow resolution.
2. Harfbuzz+python: uses harfbuzz packing and python overflow resolution. Extensions are allowed to be shared.
3. Python fallback: if harfbuzz+python runs out of resolution options, this disables extension sharing and only uses python packing. Once it succeeds control is passed back to the harfbuzz packer to produce the final packing with extension sharing enabled.
