This merges glyphsLib’s ContextualMarkFeatureWriter subclass back into
MarkFeatureWriter. This is a mechanical merge with only the bare minimum
changes to get things working. Some bug fixes and refactoring will
happen in next commits.

Check `key` is not empty before accessing first character.

…quested

We were trying to access non-existent features["mark"] when mark
feature was disabled.

Otherwise we end up with positioning statements that have duplicated
mark classes like this:

pos base a
   <anchor 200 200> mark @MC_top
   <anchor 100 200> mark @MC_top;

(one is the regular anchor, and the other is the contextual one). Which
makes no sense (feaLib shouldn’t probably allow the same mark class to
be used multiple times in the same statement).