[OS/2] define recalcXAvgCharWidth method

copied from current subsetter code (matches ufo2ft's outlineCompiler)

Lib/fontTools/ttLib/tables/O_S_2f_2.py

@@ -1,4 +1,5 @@
 from fontTools.misc import sstruct
+from fontTools.misc.roundTools import otRound
 from fontTools.misc.textTools import safeEval, num2binary, binary2num
 from fontTools.ttLib.tables import DefaultTable
 import bisect
@@ -299,6 +300,19 @@ class table_O_S_2f_2(DefaultTable.DefaultTable):
 		self.setUnicodeRanges(bits)
 		return bits
 
+	def recalcXAvgCharWidth(self, ttFont):
+		"""Recalculate xAvgCharWidth using metrics from ttFont's 'hmtx' table.
+
+		Set it to 0 if the unlikely event 'hmtx' table is not found.
+		"""
+		avg_width = 0
+		hmtx = ttFont.get("hmtx")
+		if hmtx:
+			widths = [m[0] for m in hmtx.metrics.values() if m[0] > 0]
+			avg_width = otRound(sum(widths) / len(widths))
+		self.xAvgCharWidth = avg_width
+		return avg_width
+
 
 # Unicode ranges data from the OpenType OS/2 table specification v1.7