Improve correctness of Sixel background fill

[j4james]

Summary of the Pull Request

This is an attempt to improve the correctness of the background fill
that the Sixel parser performs when an image is scrolled because it
doesn't fit on the screen.

This new behavior may not be exactly correct, but does at least match
the VT340 implementation more closely than it did before.

References and Relevant Issues

The initial Sixel parser implementation was in PR #17421.

Detailed Description of the Pull Request / Additional comments

When a Sixel image has the background select parameter set to 0 or 2, it
fills the background up to the active raster attribute dimensions prior
to writing out any actual pixel data. But this fill operation is clamped
at the boundaries of the screen, so if the image doesn't fit, and needs
to scroll, we have to perform an additional fill at that point to cover
the background of the newly revealed rows (this is something we weren't
doing before).

This later fill uses the width of the most recent raster attributes
command, which is not necessarily the same as the initial background
fill, and the height that it covers doesn't seem to take the raster
attributes into account at all. I'm not sure yet how the height is
supposed to be determined, but for now we're filling enough of the
screen to cover the sixel rows that have been output.

Validation Steps Performed

We've run a couple of tests on a real VT340, and I've confirmed that the
new implementation matches the behavior that we were seeing there.

I've also confirmed that the test case in issue #17946 now matches what
the OP was expecting, and the test case in #17887 at least works more
consistently now when scrolling (this is not what the OP was expecting
though).

PR Checklist

• Closes sixel unexpected background fill #17887
• Closes Sixel band with solid background color get transparent in some cases #17946

[j4james]

Since I'm not certain this is actually correct, I don't know if it's a good idea to merge at this point in time, but you're welcome to do so if you think it worthwhile.