James Holderness b2973eb573 Add support for the "concealed" graphic rendition attribute (#6907) ...

## Summary of the Pull Request

This PR adds support for the `SGR 8` and `SGR 28` escape sequences,
which enable and disable the _concealed/invisible_ graphic rendition
attribute. When a character is output with this attribute set, it is
rendered with the same foreground and background colors, so the text is
essentially invisible.

## PR Checklist
* [x] Closes #6876
* [x] CLA signed. 
* [x] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. Issue number
  where discussion took place: #6876

## Detailed Description of the Pull Request / Additional comments

Most of the framework for this attribute was already implemented, so it
was just a matter of updating the `TextAttribute::CalculateRgbColors`
method to make the foreground the same as the background when the
_Invisible_ flag was set. Note that this has to happen after the
_Reverse Video_ attribute is applied, so if you have white-on-black text
that is reversed and invisible, it should be all white, rather than all
black.

## Validation Steps Performed

There were already existing SGR unit tests covering this attribute in
the `ScreenBufferTests`, and the `VtRendererTest`. But I've added to the
`AdapterTest` which verifies the SGR sequences for setting and resetting
the attribute, and I've extended the `TextAttributeTests` to verify that
the color calculations return the correct values when the attribute is
set.

I've also manually confirmed that we now render the _concealed text_
values correctly in the _ISO 6429_ tests in Vttest. And I've manually
tested the output of _concealed_ when combined with other attributes,
and made sure that we're matching the behaviour of most other terminals.

2020-07-14 14:11:03 +00:00

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