This PR fixes two cases where image content wasn't correctly erased when
overwritten.
1. When legacy console APIs fill an area of the buffer using a starting
coordinate and a length, the affected area could potentially wrap over
multiple rows, but we were only erasing the overwritten image content on
the first affected row.
2. When copying an area of the buffer with text content over another
area that contained image content, the image in the target area would
sometimes not be erased, because we ignored the `_eraseCells` return
value which indicated that the image slice needed to be removed.
## References and Relevant Issues
The original code was from the Sixel implementation in PR #17421.
## Validation Steps Performed
I've manually verified that these two cases are now working as expected.
## PR Checklist
- [x] Closes#18568
When we have a series of image slices of differing widths, which also
don't align with the cell boundaries, we can get rounding errors in the
scaling which makes the different slices appear misaligned.
This PR fixes the issue by removing the 4 pixel width alignment that was
enforced in the `ImageSlice` class, since that's not actually necessary
when the pixels themselves are already 4 bytes in size. And without
that, the widths should be correctly aligned with the cell boundaries.
## References and Relevant Issues
The initial Sixel implementation was added in PR #17421.
## Validation Steps Performed
I've confirmed that this fixes the rendering glitches reported in
#17711, and all my existing Sixel tests still work as expected.
Closes#17711
* Add a revision to `ImageSlice` so that the renderers
can use it to cache them as bitmaps across frames.
* Hooked up the revision tracking to AtlasEngine to cache the
slices into `Buffer`s so we can own them into the `Present`.
* Hooked up those snapshots to BackendD3D with a straightforward
hashmap -> atlas-rect logic. Just like rendering text.
* Hooked up BackendD2D with a bad, but simple & direct drawing logic.
* Bonus: Modify `ImageSlice` to be returned as a raw pointers
as this helps performance slightly. (Trivial type == good.)
* Bonus: Fixed the `_debugShowDirty` code (disabled by default).
## Validation Steps Performed
* `mpv --really-quiet --vo=sixel foo.mp4` looks good ✅
* Scroll up down & observe dirty rects ✅
## 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.
---------
Co-authored-by: Dustin L. Howett <dustin@howett.net>
