Add support for resetting the color scheme with RIS

[j4james]

Summary of the Pull Request

This improves our RIS (hard reset) implementation, so it now also
resets any changes that are made to the color table and color aliases,
which is one of the things it's supposed to be doing.

References and Relevant Issues

This is also a small step towards implementing the OSC sequences that
reset individual color table entries (issue #3719).

Detailed Description of the Pull Request / Additional comments

The way this works is by having a second copy of the color table and
alias indices to hold the default values in the RenderSettings class.
This default set is initially populated at startup with the user's
chosen color scheme, but can also potentially be updated if the user
changes their settings while a session is already in progress.

When we receive an RIS request, we just copy the default values back
over the active settings, and refresh the renderer.

Validation Steps Performed

I've manually tested both OpenConsole and Windows Terminal by changing
my color scheme programmatically, and then confirming that the original
colors are restored when an RIS sequence is received.

I've also added some basic unit tests that check both the color aliases
and color table are restored by RIS.

PR Checklist

• Tests added/passed

[lhecker]

If the code above calls SetColorTableEntry, won't this overwrite the modified colors set by the terminal application (as if a RIS was received)?

[j4james]

Yeah. That's typically what you want if you're actually using the settings to change the color scheme, but not so much when you're editing an unrelated setting. In fact the colors can even end up being reset when when you haven't edited the settings at all! This is a known problem that is tracked in issue #11522.

[lhecker]

Yeah. That's typically what you want if you're actually using the settings to change the color scheme

I'm somewhat surprised about that - I expected the opposite. But that answered my question. It was the only concern I had, and otherwise the PR looks great. :)

[j4james]

Just to be clear, I don't feel strongly about it either way, but that's the way it's always worked, and it seemed to me a reasonable behavior. If you open up a shell with a particular color scheme, decide you don't like it, so you change the scheme in the settings, wouldn't you expect that shell to update straight away?

Of course if you were busy using an application that set its own palette, having it overwritten would be annoying, but it's much less likely you'd be trying to switch your color scheme in that situation (this assumes that #11522 is fixed, so changing other settings wouldn't trigger a color scheme refresh).

Either way, if we want to reconsider that behavior, I think it's best raised as a separate issue.

[DHowett]

This is excellent, thank you