subset: 'draw' charstrings to set 'width' before calling 'drop_hints'
subset: 'draw' charstrings to set 'width' before calling 'drop_hints'

the horizontal advance of hinted charstrings gets lost when the CFF hinting
is stripped with '--no-hinting' option, as noted by @miguelsousa here:

https://github.com/behdad/fonttools/issues/343#issuecomment-156234918

The reason is T2CharString objects only get assigned a 'witdh' attribute
after their 'draw' method is called.

The subsetter's 'drop_hints' function attempts to insert the width back
at the beginning of the de-hinted charstring's program, but can do that
only if the charstring does have a 'width' attribute:

https://github.com/behdad/fonttools/blob/master/Lib/fontTools/subset/__init__.py#L1908

googlefonts/fonttools,fonttools/fonttools