[merge] use recalcXAvgCharWidth method on merged font

Fixes #2538, supersedes #2590

Lib/fontTools/merge/__init__.py

@@ -155,6 +155,11 @@ class Merger(object):
 	def _postMerge(self, font):
 		layoutPostMerge(font)
 
+		if "OS/2" in font:
+			# https://github.com/fonttools/fonttools/issues/2538
+			# TODO: Add an option to disable this?
+			font["OS/2"].recalcXAvgCharWidth(font)
+
 
 __all__ = [
 	'Options',

Lib/fontTools/merge/tables.py

@@ -132,7 +132,7 @@ ttLib.getTableClass('OS/2').mergeMap = {
 	'*': first,
 	'tableTag': equal,
 	'version': max,
-	'xAvgCharWidth': avg_int, # Apparently fontTools doesn't recalc this
+	'xAvgCharWidth': first, # Will be recalculated at the end on the merged font
 	'fsType': mergeOs2FsType, # Will be overwritten
 	'panose': first, # FIXME: should really be the first Latin font
 	'ulUnicodeRange1': bitwise_or,

Tests/merge/data/CFFFont_expected.ttx

@@ -834,7 +834,7 @@
     <!-- The fields 'usFirstCharIndex' and 'usLastCharIndex'
          will be recalculated by the compiler -->
     <version value="4"/>
-    <xAvgCharWidth value="505"/>
+    <xAvgCharWidth value="501"/>
     <usWeightClass value="400"/>
     <usWidthClass value="5"/>
     <fsType value="00000000 00000000"/>