This PR adds support for the ANSI _doubly underlined_ graphic rendition
attribute, which is enabled by the `SGR 21` escape sequence.
There was already an `ExtendedAttributes::DoublyUnderlined` flag in the
`TextAttribute` class, but I needed to add `SetDoublyUnderlined` and
`IsDoublyUnderlined` methods to access that flag, and update the
`SetGraphicsRendition` methods of the two dispatchers to set the
attribute on receipt of the `SGR 21` sequence. I also had to update the
existing `SGR 24` handler to reset _DoublyUnderlined_ in addition to
_Underlined_, since they share the same reset sequence.
For the rendering, I've added a new grid line type, which essentially
just draws an additional line with the same thickness as the regular
underline, but slightly below it - I found a gap of around 0.05 "em"
between the lines looked best. If there isn't enough space in the cell
for that gap, the second line will be clamped to overlap the first, so
you then just get a thicker line. If there isn't even enough space below
for a thicker line, we move the offset _above_ the first line, but just
enough to make it thicker.
The only other complication was the update of the `Xterm256Engine` in
the VT renderer. As mentioned above, the two underline attributes share
the same reset sequence, so to forward that state over conpty we require
a slightly more complicated process than with most other attributes
(similar to _Bold_ and _Faint_). We first check whether either underline
attribute needs to be turned off to send the reset sequence, and then
check individually if each of them needs to be turned back on again.
## Validation Steps Performed
For testing, I've extended the existing attribute tests in
`AdapterTest`, `VTRendererTest`, and `ScreenBufferTests`, to make sure
we're covering both the _Underlined_ and _DoublyUnderlined_ attributes.
I've also manually tested the `SGR 21` sequence in conhost and Windows
Terminal, with a variety of fonts and font sizes, to make sure the
rendering was reasonably distinguishable from a single underline.
Closes#2916
e7a1a675af