Add support for the Presentation State reports

[j4james]

This PR introduces two new sequences, DECRQPSR and DECRSPS, which
provide a way for applications to query and restore the presentation
state reports. This includes the tab stop report (DECTABSR) and the
cursor information report (DECCIR).

One part of the cursor information report contains the character set
designations and mapped G-sets. But we weren't tracking that data in a
way that could easily be reported, so I needed to do some refactoring in
the TerminalOutput class to make that accessible.

Other than that, the rest was fairly straightforward. It was just a
matter of packaging up all the information into the correct format for
the returned DCS string, and in the case of the restore operations,
parsing the incoming data and applying the new state.

Validation Steps Performed

Thanks to @al20878, we were able to test these operations on a real
VT525, and I've manually verified that our implementation matches that
behavior. I've also added some unit tests covering both reports.

Closes #14984

[j4james]

I think the remaining audit failures are an existing issue - you'll see the same errors in a number of recent PRs. I've submitted a fix in PR #15002.

[DHowett]

This is clever (this whole function, despite me marking just one line) and I bet @lhecker has thoughts about it 😁

[DHowett]

It's pretty much a functor that has been degranulated into a lambda with captured state!

[lhecker]

I haven't considered writing a state machine this way yet. This is pretty neat technique!

[lhecker]

It'd be possible to turn VTID into a:

union VTID
{
private:
    uint64_t _value = 0;
    char _string[8];
};

and it'd be entirely valid C++. That way we could get the string out of it in constant time and simplify both string related functions to a simple std::copy_n for FromString and return &_string[0] for the ToString function. All we need to do to make this work is to ensure that _string[7] is 0 in the 2 constructors this class has, which I feel like is a feasible modification to do (i.e. via _value = value & 0x00FFFFFFFFFFFFFF).

[lhecker]

I realized that this won't work because constexpr doesn't support reinterpret_cast or equivalents (like unions). So here's another alternative:

class VTID
{
public:
    template<size_t Length>
    consteval VTID(const char (&s)[Length]) :
        _value{ _FromString(s) }
    {
    }

    constexpr VTID(const uint64_t value) :
        // Ensure we got a trailing 0 for our string conversion.
        _value{ value & 0x00FFFFFFFFFFFFFF }
    {
    }

private:
    template<size_t Length>
    static consteval uint64_t _FromString(const char (&s)[Length])
    {
        // Length already includes the trailing 0 of the string literal.
        static_assert(Length <= sizeof(_value));

        char buffer[sizeof(_value)]{};
        std::copy_n(&s[0], Length, &buffer[0]);

        return std::bit_cast<uint64_t>(buffer);
    }

    uint64_t _value;
};

I should mention that this is just an idea and doesn't necessarily mean that I'm convinced it's a good one. 😅

[j4james]

Yeah, I think this is good idea. I'll give it a try.

[j4james]

In the end I just used your union trick to get a more efficient ToString and left the rest more or less as it was (other than making sure there was always a null terminator).

I had initially thought I could make the ToString return a char*, but the fmt::format call couldn't cope with that (I'm assuming because of the mismatch between char and wchar_t). It was happy with a string_view though.

It's maybe not perfect, but I think this is an improvement on the initial implementation.

[lhecker]

Reading through my comments again, I noticed they're all just minor nits. Please feel free to address them, but I'm also good with the PR as is. 🙂

[DHowett]

I love the new VTID.