mesa: implement the last of GL_ARB_debug_output

Store client-defined message IDs in a hash table,
and sort them by severity into three linked lists
so they can be selected by severity level later.

src/mesa/main/context.c

@@ -1138,6 +1138,8 @@ _mesa_free_context_data( struct gl_context *ctx )
    /* needs to be after freeing shared state */
    _mesa_free_display_list_data(ctx);
 
+   _mesa_free_errors_data(ctx);
+
    if (ctx->Extensions.String)
       free((void *) ctx->Extensions.String);