CFF: set FontMatrix based on unitsPerEm automatically, unless manually overridden

Lib/fontTools/fontBuilder.py

@@ -478,6 +478,9 @@ class FontBuilder(object):
         topDict.Private = private
         for key, value in fontInfo.items():
             setattr(topDict, key, value)
+        if "FontMatrix" not in fontInfo:
+            scale = 1 / self.font["head"].unitsPerEm
+            topDict.FontMatrix = [scale, 0, 0, scale, 0, 0]
 
         charStrings = CharStrings(None, topDict.charset, globalSubrs, private, fdSelect, fdArray)
         for glypnName, charString in charStringsDict.items():

Tests/fontBuilder/data/test.otf.ttx

@@ -161,7 +161,7 @@
       <UnderlineThickness value="50"/>
       <PaintType value="0"/>
       <CharstringType value="2"/>
-      <FontMatrix value="0.001 0 0 0.001 0 0"/>
+      <FontMatrix value="0.0009765625 0 0 0.0009765625 0 0"/>
       <FontBBox value="100 100 500 1000"/>
       <StrokeWidth value="0"/>
       <!-- charset is dumped separately as the 'GlyphOrder' element -->