[truetype] Make blend/design coordinate round-tripping work.

Behdad reported that setting blend coordinates, then getting design
coordinates did incorrectly return the default instance's
coordinates.

* src/truetype/ttgxvar.c (tt_set_mm_blend): Fix it.

ChangeLog

@@ -1,3 +1,13 @@
+2018-01-03  Werner Lemberg  <wl@gnu.org>
+
+	[truetype] Make blend/design coordinate round-tripping work.
+
+	Behdad reported that setting blend coordinates, then getting design
+	coordinates did incorrectly return the default instance's
+	coordinates.
+
+	* src/truetype/ttgxvar.c (tt_set_mm_blend): Fix it.
+
 2017-12-31  Werner Lemberg  <wl@gnu.org>
 
 	* src/sfnt/ttcmap.c (tt_cmap2_char_next): Fix endless loop.

src/truetype/ttgxvar.c

@@ -2493,14 +2493,14 @@
                  coords,
                  num_coords * sizeof ( FT_Fixed ) );
 
+    face->doblend = TRUE;
+
     if ( set_design_coords )
       ft_var_to_design( face,
                         all_design_coords ? blend->num_axis : num_coords,
                         blend->normalizedcoords,
                         blend->coords );
 
-    face->doblend = TRUE;
-
     if ( face->cvt )
     {
       switch ( manageCvt )