Implemented IME preview text for CJK characters in SDL2 backend

[katemonster33]

Added support for displaying the IME preview text in an InputText field when the backend requests it to be shown. ( #5878 )

The internal buffers for InputText are not modified, the behavior currently only works for single-line InputText fields.

[katemonster33]

No interest here? I don't like leaving PRs open in my list, i will close this in a few days if there's no activity

[ocornut]

There is always interest unless closed. I prefer to keep PR open to track useful ideas and references. It most often takes a long time until things gets fixed/finished and merged so I am unlikely to finish that work soon. If you get impatient feel free to close that but that’s essentially retracting your (legit/viable) idea from the pool of ideas.

I noticed you closing issues before I have a chance to get through them, which is unfortunate but my resources are limited.

[katemonster33]

Ok I just wasn't familiar with your workflow here

[seagetch]

I have backported this patch to old version of ImGui on MacOS, and found a problem.
When I pressed enter to confirm the pre-edit text in Japanese, all pre-edit text is removed.
It seems enter key event is sent first in MacOS.

I added following code in KEYDOWN/KEYUP event, and it worked fine in MacOS.

---

    case SDL_KEYDOWN:
    case SDL_KEYUP:
    {
        if (event->key.keysym.sym == SDLK_RETURN && strlen(io.PreEditText) > 0) {
            // Ignore keyevent when preedit text exists.
            return false;
        }

---

And it worked fine in Ubuntu Linux 22.4 without any additional patches, with fcitx.