vbo: fix initial value of ctx->Driver.CurrentSavePrimitive

This is set during context creation/initialization.  We know we're
not inside glBegin/glEnd at this point so use PRIM_OUTSIDE_BEGIN_END.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/vbo/vbo_save.c

@@ -86,7 +86,7 @@ void vbo_save_init( struct gl_context *ctx )
       }
    }
 
-   ctx->Driver.CurrentSavePrimitive = PRIM_UNKNOWN;
+   ctx->Driver.CurrentSavePrimitive = PRIM_OUTSIDE_BEGIN_END;
 }