mesa: allow for extra per-context init

(cherry picked from commit 815cdcfbc0)

src/mesa/main/context.h

@@ -114,6 +114,9 @@ _mesa_initialize_context( GLcontext *ctx,
                           const struct dd_function_table *driverFunctions,
                           void *driverContext );
 
+extern void
+_mesa_initialize_context_extra(GLcontext *ctx);
+
 extern void
 _mesa_free_context_data( GLcontext *ctx );