[Snippets] Add script for cmap subtable format conversion

Fixes https://github.com/behdad/fonttools/issues/340

Snippets/cmap-format.py

@@ -0,0 +1,40 @@
+#! /usr/bin/env python
+
+# Sample script to convert legacy cmap subtables to format-4
+# subtables.  Note that this is rarely what one needs.  You
+# probably need to just drop the legacy subtables if the font
+# already has a format-4 subtable.
+#
+# Other times, you would need to convert a non-Unicode cmap
+# legacy subtable to a Unicode one.  In those cases, use the
+# getEncoding() of subtable and use that encoding to map the
+# characters to Unicode...  TODO: Extend this script to do that.
+
+from __future__ import print_function, division, absolute_import
+from fontTools.misc.py23 import *
+from fontTools.ttLib import TTFont
+from fontTools.ttLib.tables._c_m_a_p import CmapSubtable
+import sys
+
+if len(sys.argv) != 3:
+	print("usage: cmap-format.py fontfile.ttf outfile.ttf")
+	sys.exit(1)
+fontfile = sys.argv[1]
+outfile = sys.argv[2]
+font = TTFont(fontfile)
+
+cmap = font['cmap']
+outtables = []
+for table in cmap.tables:
+	if table.format in [4, 12, 13, 14]:
+		outtables.append(table)
+	# Convert ot format4
+	newtable = CmapSubtable.newSubtable(4)
+	newtable.platformID = table.platformID
+	newtable.platEncID = table.platEncID
+	newtable.language = table.language
+	newtable.cmap = table.cmap
+	outtables.append(newtable)
+cmap.tables = outtables
+
+font.save(outfile)