[indic] Use mutable for virama_glyph

src/hb-ot-shape-complex-indic.cc

@@ -508,7 +508,7 @@ struct indic_shape_plan_t
 
       /* Our get_nominal_glyph() function needs a font, so we can't get the virama glyph
        * during shape planning...  Instead, overwrite it here.  It's safe.  Don't worry! */
-      (const_cast<indic_shape_plan_t *> (this))->virama_glyph = glyph;
+      virama_glyph = glyph;
     }
 
     *pglyph = glyph;
@@ -518,7 +518,7 @@ struct indic_shape_plan_t
   const indic_config_t *config;
 
   bool is_old_spec;
-  hb_codepoint_t virama_glyph;
+  mutable hb_codepoint_t virama_glyph;
 
   would_substitute_feature_t rphf;
   would_substitute_feature_t pref;