XTRA + slnt outline issue in C, D, ...

A user wrote to GF that the `C` and `D` and maybe more have a point outline issue with slnt + weight + opsz + XTRA:

> I am getting back to you as we are still facing issues with the Roboto Flex. While testing it on the [Slice app](https://slice-gui.netlify.app/) you gave us, we realized that there is a problem within the font itself. We then went on Google Fonts, and faced the same problem, meaning the problem is with the font itself.
>
> You can see the problem in the screenshot attached: We need to provide our client a customized Roboto file, but when we intervene on the Xtra parameter, (directly on Google fonts in this case) the font results broken.
>
> We are in a difficult position with our client as we already “sold” the font, but at the moment it turns out to be impossible to deliver anything..
> This problem is adds up on the previous problem of compatibility that I wrote you about last week, and unfortunately this is affecting our credibility with the client.
>
> Do you think it’s possible to resolve it quickly?
>
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