Mute FVal E5200 (based on outdated OT spec) for var fonts.

(issue fonttools#2109)

Lib/fontbakery/specifications/general.py

@@ -190,7 +190,18 @@ def com_google_fonts_check_037(font):
     # contours because they are used to draw each portion
     # of variable glyph features.
     "Intersecting contours",
-    "Intersecting components of composite glyph"
+    "Intersecting components of composite glyph",
+
+    # DeltaFormat = 32768 (same as 0x8000) means VARIATION_INDEX,
+    # according to https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2
+    # The FontVal problem description for this check (E5200) only mentions
+    # the other values as possible valid ones. So apparently this means FontVal
+    # implementation is not up-to-date with more recent versions of the OpenType spec
+    # and that's why these spurious FAILs are being emitted.
+    # That's good enough reason to mute it.
+    # More info at:
+    # https://github.com/googlefonts/fontbakery/issues/2109
+    "The device table's DeltaFormat value is invalid"
   ]
 
   from fontTools.ttLib import TTFont