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

Lib/fontTools/subset/__init__.py

@@ -7,7 +7,7 @@ from fontTools.misc.py23 import *
 from fontTools import ttLib
 from fontTools.ttLib.tables import otTables
 from fontTools.misc import psCharStrings
-from fontTools.pens.boundsPen import BoundsPen
+from fontTools.pens.basePen import NullPen
 from fontTools.misc.loggingTools import Timer
 import sys
 import struct
@@ -1847,7 +1847,7 @@ def prune_pre_subset(self, font, options):
                 private = font.Private
             dfltWdX = private.defaultWidthX
             nmnlWdX = private.nominalWidthX
-            pen = BoundsPen(None)
+            pen = NullPen()
             c.draw(pen)  # this will set the charstring's width
             if c.width != dfltWdX:
                 c.program = [c.width - nmnlWdX, 'endchar']
@@ -2207,7 +2207,9 @@ def prune_post_subset(self, options):
                 subrs = getattr(c.private, "Subrs", [])
                 decompiler = _DehintingT2Decompiler(css, subrs, c.globalSubrs)
                 decompiler.execute(c)
+            pen = NullPen()
             for charstring in css:
+                charstring.draw(pen)  # this will set the charstring's width
                 charstring.drop_hints()
             del css