Less hacky double-rendering prevention in mapped task

[uranusjr]

Instead of inventing a separate way to track what XComArg has been rendered (due to task unmapping), we can actually track this with seen_oids. This makes the mechanism considerably less hacky.

[potiuk]

It does look quite a bit less magical.

I do not follow all aspects there and cannot easily go through all scenarios, but this change looks very solid and seems to keep all the properties of the previous approach in much cleaner way, so LGTM.