[subset] VVAR fails to subset with --retain-gids

[blueshade7]

A variable font with VVAR table fails to subset with --retain-gids option.

Font file attached:
SourceSerifVariable-Roman-VVAR.abc.ttf.zip

% pyftsubset SourceSerifVariable-Roman-VVAR.abc.ttf --text=ac --retain-gids --output-file=output.ttf
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/2.7/bin/pyftsubset", line 11, in <module>
    load_entry_point('fonttools==3.39.1.dev0', 'console_scripts', 'pyftsubset')()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/misc/loggingTools.py", line 375, in wrapper
    return func(*args, **kwds)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/subset/__init__.py", line 2832, in main
    save_font(font, outfile, options)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/misc/loggingTools.py", line 375, in wrapper
    return func(*args, **kwds)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/subset/__init__.py", line 2679, in save_font
    font.save(outfile, reorderTables=options.canonical_order)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/ttFont.py", line 173, in save
    writer_reordersTables = self._save(tmp)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/ttFont.py", line 212, in _save
    self._writeTable(tag, writer, done, tableCache)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/ttFont.py", line 633, in _writeTable
    tabledata = self.getTableData(tag)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/ttFont.py", line 651, in getTableData
    return self.tables[tag].compile(self)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/tables/otBase.py", line 70, in compile
    self.table.compile(writer, font)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/tables/otBase.py", line 709, in compile
    conv.write(writer, font, table, value)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/tables/otConverters.py", line 520, in write
    value.compile(subWriter, font)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/tables/otBase.py", line 649, in compile
    table = self.preWrite(font)
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/fonttools-3.39.1.dev0-py2.7.egg/fontTools/ttLib/tables/otTables.py", line 632, in preWrite
    mapping = [mapping[g] for g in glyphOrder]
KeyError: ('b', 'VarIdxMap')

[readroberts]

I suspect that this just needs the same fix as was applied for HVAR in commit 6355376 on March 19.