[subset] Update PairPosFormat2 subsetting to match fontTools updated subsetting (https://github.com/fonttools/fonttools/pull/2221) - subset class def 1 against the coverage table. - Don't re-use class 0 in class def 2. - Ignore class 0 glyphs for the purposes of determing format 1 vs format 2 encoding for ClassDef. Add an additional test case which demonstrates these issues.