docstr: Returns overhead in bytes of encoding this characteristic
  as a VarData.
code: def _characteristic_overhead(columns):
      """Returns overhead in bytes of encoding this characteristic
      as a VarData."""
      c = 4 + 6  # 4 bytes for LOffset, 6 bytes for VarData header
      c += bit_count(columns) * 2
      return c
  
imports: from fontTools.misc.roundTools import noRound, otRound
  from fontTools.misc.intTools import bit_count
  from fontTools.ttLib.tables import otTables
  from fontTools.varLib.models import supportScalar
  from fontTools.varLib.builder import buildVarRegionList, buildVarStore, buildVarRegion, buildVarData
  from functools import partial
  from collections import defaultdict
  from heapq import heappush, heappop
label_desc: Use image node_id 8 for calling the _Encoding obj's underlying member method code with example usage: obj._characteristic_overhead(columns) and returns: c