Implemented path operators in a new struct PathProc hooked up to CSOpSet
Added current point to CSInterpEnv

as they aren't instantiated with BlendArg (yet)

Templatized ArgStack so it may store the default value along with blend deltas as BlendArg while parsing blend operator arguments in CFF2 charstring
Added get_region_count() method to VarRegionList & VariationStore

renamed confusing Stack.size to Stack.count

Factored out CFF1 & CFF2 common subsetting code in hb-subset-cff-common.hh

Subr-flattened charstrings are temporarily re-encoded in ByteStrBuff during "plan" phase, then copied to hb_serialize_context_t during "write" phase

CSOpSet may callback opcode processing "virtual" functions via CRTP

Numer struct may store a value as fixed optionally in addition to int and float

along with CFF2 charstring interpreter
factored out common code between CFF1 & CFF2 to CSInterpreter
moved fetch_op from Interpreter to InterpEnv
misc code clean up & bug fixes