[truetype] Reload context after re-executing `prep`.

When a different hinting mode from the current is selected, the `prep` table
must be re-executed with the new mode.  After this happens the context must
be re-loaded in preparation for the glyph program to be run.

Fixes #1104.

* truetype/ttgload.c (tt_loader_init): Add call to `TT_Load_Context`.

src/truetype/ttgload.c

@@ -2717,6 +2717,9 @@
         error = tt_size_run_prep( size, pedantic );
         if ( error )
           return error;
+        error = TT_Load_Context( exec, face, size );
+        if ( error )
+          return error;
       }
 
       /* check whether the cvt program has disabled hinting */