[varLib] include default maps for all avar axes, even without <map> e…

…lements

This is a follow-up to Jens' comment:

19c4b37#commitcomment-23458151

Now, if there's any axis that has 'interesting' segment maps (and thus
an avar table is added), we also ensure that for the rest of the axes
that aren't modified (either because no <map> elements are defined or
because an identity mapping is defined in the designspace), we always
have a non-empty segment maps array containing the three default maps:
{-1.0: -1.0, 0.0: 0.0, 1.0: 1.0}.

This is to work around CoreText and DirectWrite rendering issue with
empty avar segment maps arrays.

Lib/fontTools/varLib/__init__.py

@@ -109,7 +109,14 @@ def _add_avar(font, axes):
 
 	interesting = False
 	for axis in axes.values():
-		curve = avar.segments[axis.tag] = {}
+		# Currently, some rasterizers require that the default value maps
+		# (-1 to -1, 0 to 0, and 1 to 1) be present for all the segment
+		# maps, even when the default normalization mapping for the axis
+		# was not modified.
+		# https://github.com/googlei18n/fontmake/issues/295
+		# https://github.com/fonttools/fonttools/issues/1011
+		# TODO(anthrotype) revert this (and 19c4b37) when issue is fixed
+		curve = avar.segments[axis.tag] = {-1.0: -1.0, 0.0: 0.0, 1.0: 1.0}
 		if not axis.map:
 			continue
 
@@ -135,16 +142,6 @@ def _add_avar(font, axes):
 		keys = [models.normalizeValue(v, keys_triple) for v in keys]
 		vals = [models.normalizeValue(v, vals_triple) for v in vals]
 
-		# Work around rendering issue with CoreText when avar table
-		# contains segment maps with zero axis value maps.
-		# Currently, CoreText requires that the three default value maps
-		# (-1 to -1, 0 to 0, and 1 to 1) be present for all the segment
-		# maps, even when the default normalization mapping for the axis
-		# was not modified.
-		# https://github.com/googlei18n/fontmake/issues/295
-		# TODO(anthrotype) revert this change once CoreText is modified
-		curve.update({-1.0: -1.0, 0.0: 0.0, 1.0: 1.0})
-
 		if all(k == v for k, v in zip(keys, vals)):
 			continue
 		interesting = True