[truetype] Make `TT_Set_MM_Blend' idempotent (#54388).

* src/truetype/ttgxvar.c (tt_set_mm_blend): Correctly set
`face->doblend' if the current call to the function yields the same
blend coordinates as the previous call.

ChangeLog

@@ -1,3 +1,11 @@
+2018-07-27  Werner Lemberg  <wl@gnu.org>
+
+	[truetype] Make `TT_Set_MM_Blend' idempotent (#54388).
+
+	* src/truetype/ttgxvar.c (tt_set_mm_blend): Correctly set
+	`face->doblend' if the current call to the function yields the same
+	blend coordinates as the previous call.
+
 2018-07-27  Werner Lemberg  <wl@gnu.org>
 
 	[psaux, type1]: More tracing improvements.

src/truetype/ttgxvar.c

@@ -2555,7 +2555,11 @@
 
       /* return value -1 indicates `no change' */
       if ( !have_diff )
+      {
+        face->doblend = TRUE;
+
         return -1;
+      }
 
       for ( ; i < mmvar->num_axis; i++ )
       {