Simplify ExtSubTable
Seprate longOffset from ExtSubTable
Rename repeatOffset to aux

To be used for other purposes soon.
Revert c386d54fe3f1bb12561126044cc5a7a7f0d80fe2
Implement GDEF MarkGlyphSetsDef

Aka. "Mark filtering sets".

Fixes https://github.com/behdad/fonttools/issues/39
[subset] Support GDEF MarkGlyphSetsDef
[subset] Drop GSUB/GPOS main list items if empty