## Summary of the Pull Request
This PR introduces basic support for the Sixel graphics protocol in
conhost, limited to the GDI renderer.
## References and Relevant Issues
This is a first step towards supporting Sixel graphics in Windows
Terminal (#448), but that will first require us to have some form of
ConPTY passthrough (#1173).
## Detailed Description of the Pull Request / Additional comments
There are three main parts to the architecture:
* The `SixelParser` class takes care of parsing the incoming Sixel `DCS`
sequence.
* The resulting image content is stored in the text buffer in a series
of `ImageSlice` objects, which represent per-row image content.
* The renderer then takes care of painting those image slices for each
affected row.
The parser is designed to support multiple conformance levels so we can
one day provide strict compatibility with the original DEC hardware. But
for now the default behavior is intended to work with more modern Sixel
applications. This is essentially the equivalent of a VT340 with 256
colors, so it should still work reasonably well as a VT340 emulator too.
## Validation Steps Performed
Thanks to the work of @hackerb9, who has done extensive testing on a
real VT340, we now have a fairly good understanding of how the original
Sixel hardware terminals worked, and I've tried to make sure that our
implementation matches that behavior as closely as possible.
I've also done some testing with modern Sixel libraries like notcurses
and jexer, but those typically rely on the terminal implementing certain
proprietary Xterm query sequences which I haven't included in this PR.
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Co-authored-by: Dustin L. Howett <dustin@howett.net>
