[cff, truetype] Always set up default variable instance.

For default variable instances `cff_face_init` did not set the blend.  This
mostly worked as later use of the unset blend produced the default
variation.  However, if a user called `TT_Get_MM_Var` the blend would be
partially set up, but not fully.  In particular the number of axes, the axis
definitions, and the instance locations would be set up, but not the current
instance location (`coords` and `normalizedcoords`).  This could lead to the
default instances of CFF2 fonts erroring on any use of `blend`.

Ensure the default variable instance is fully set up by always calling
`FT_Set_Named_Instance` on a variable face.

* src/cff/cffobjs.c (cff_face_init): Call `FT_Set_Named_Instance` on
default instances.
* src/truetype/ttobjs.c (tt_face_init): Ditto.

Fixes #1268.

src/cff/cffobjs.c

@@ -694,8 +694,7 @@
         FT_UInt  instance_index = (FT_UInt)face_index >> 16;
 
 
-        if ( FT_HAS_MULTIPLE_MASTERS( cffface ) &&
-             instance_index > 0                 )
+        if ( FT_HAS_MULTIPLE_MASTERS( cffface ) )
         {
           error = FT_Set_Named_Instance( cffface, instance_index );
           if ( error )

src/truetype/ttobjs.c

@@ -784,8 +784,7 @@
       FT_UInt  instance_index = (FT_UInt)face_index >> 16;
 
 
-      if ( FT_HAS_MULTIPLE_MASTERS( ttface ) &&
-           instance_index > 0                )
+      if ( FT_HAS_MULTIPLE_MASTERS( ttface ) )
       {
         error = FT_Set_Named_Instance( ttface, instance_index );
         if ( error )