Bug#39200: optimize table does not recognize ROW_FORMAT=COMPRESSED

When doing ALTER TABLE, we forgot to point out that we actually have
ROW_FORMAT information (from the original table), so we dropped to
"sensible defaults". This affects both ALTER TABLE and OPTIMIZE TABLE
which may fall back on ALTER TABLE for InnoDB.

We now flag that we do indeed know the row-type, thereby preserving
compression-type etc.

No .test in 5.1 since we'd need a reasonable new plugin from InnoDB to
show this properly; in higher versions, maria can demonstrate this.

sql/sql_table.cc:
  In mysql_alter_table() flag that we have row-type info
  from old table. In compare_tables(), we must explicitly
  check whether row-type has changed (rather than rely on
  the flag which will always be set at this point now).

sql/sql_table.cc

@@ -5314,7 +5314,7 @@ compare_tables(TABLE *table,
       create_info->used_fields & HA_CREATE_USED_ENGINE ||
       create_info->used_fields & HA_CREATE_USED_CHARSET ||
       create_info->used_fields & HA_CREATE_USED_DEFAULT_CHARSET ||
-      create_info->used_fields & HA_CREATE_USED_ROW_FORMAT ||
+      (table->s->row_type != create_info->row_type) ||
       create_info->used_fields & HA_CREATE_USED_PACK_KEYS ||
       create_info->used_fields & HA_CREATE_USED_MAX_ROWS ||
       (alter_info->flags & (ALTER_RECREATE | ALTER_FOREIGN_KEY)) ||
@@ -6307,7 +6307,10 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   }
 
   if (create_info->row_type == ROW_TYPE_NOT_USED)
+  {
     create_info->row_type= table->s->row_type;
+    create_info->used_fields |= HA_CREATE_USED_ROW_FORMAT;
+  }
 
   DBUG_PRINT("info", ("old type: %s  new type: %s",
              ha_resolve_storage_engine_name(old_db_type),