fontBuilder: Allow varLib to use fresh CFF table

Make sure the CFF table generated by fontBuilder can be used by varLib
without having to compile and decompile the table first. This was
breaking in converting the CFF table to CFF2 due to some unset
attributes.

Lib/fontTools/fontBuilder.py

@@ -506,6 +506,7 @@ class FontBuilder(object):
         fontSet = CFFFontSet()
         fontSet.major = 1
         fontSet.minor = 0
+        fontSet.otFont = self.font
         fontSet.fontNames = [psName]
         fontSet.topDictIndex = TopDictIndex()
 
@@ -520,6 +521,7 @@ class FontBuilder(object):
         topDict = TopDict()
         topDict.charset = self.font.getGlyphOrder()
         topDict.Private = private
+        topDict.GlobalSubrs = fontSet.GlobalSubrs
         for key, value in fontInfo.items():
             setattr(topDict, key, value)
         if "FontMatrix" not in fontInfo:

Tests/fontBuilder/fontBuilder_test.py

@@ -255,6 +255,19 @@ def test_build_cff2(tmpdir):
     _verifyOutput(outPath)
 
 
+def test_build_cff_to_cff2(tmpdir):
+    fb, _, _ = _setupFontBuilder(False, 1000)
+
+    pen = T2CharStringPen(600, None)
+    drawTestGlyph(pen)
+    charString = pen.getCharString()
+    charStrings = {".notdef": charString, "A": charString, "a": charString, ".null": charString}
+    fb.setupCFF("TestFont", {}, charStrings, {})
+
+    from fontTools.varLib.cff import convertCFFtoCFF2
+    convertCFFtoCFF2(fb.font)
+
+
 def test_setupNameTable_no_mac():
     fb, _, nameStrings = _setupFontBuilder(True)
     fb.setupNameTable(nameStrings, mac=False)