add Platform IME Data Export

[KarryCharon]

Export the PlatformImeData struct into python.

• This structure is useful for input method candidates box positoin sync.
• Such as some input action occurs, but the input method candidates box will display at the right bottom of screen.
• Via this one, we can do some position changing.

[nurpax]

Do you have some examples from other projects how this is used? I'm quite unfamiliar with this feature, and I'm a little wary exposing anything that I don't know their intended usage for. I couldn't find any examples of PlatformImeData in imgui's own codebase, other than in the backends.

I also had trouble accessing the Imgui context fields directly and refactored the code earlier to not access the fields to get the docking branch to build. This was the change: 5f9469b. I'm a little worried that those build problems would make a comeback, as there seems to be no official accessor function for getting the ImeData instance.

[KarryCharon]

Do you have some examples from other projects how this is used? I'm quite unfamiliar with this feature, and I'm a little wary exposing anything that I don't know their intended usage for. I couldn't find any examples of PlatformImeData in imgui's own codebase, other than in the backends.

I also had trouble accessing the Imgui context fields directly and refactored the code earlier to not access the fields to get the docking branch to build. This was the change: 5f9469b. I'm a little worried that those build problems would make a comeback, as there seems to be no official accessor function for getting the ImeData instance.

Without IME compositing position sync, it would be like this:

20260109155241_rec_.mp4

But if we have some IME input text position info, we will got this:

20260109155627_rec_.mp4

[KarryCharon]

There is also some official backend example for text input box sync: https://github.com/ocornut/imgui/blob/7b3ad4a282dc6f5007c49d35853e32f364e3e3d7/backends/imgui_impl_sdl3.cpp#L202

[nurpax]

Reading the imgui source code, it looks like the IME is accessed via a callback mechanism where Imgui calls a backend callback. For example, in the ImGui::EndFrame(), I see this code:

    // Notify Platform/OS when our Input Method Editor cursor has moved (e.g. CJK inputs using Microsoft IME)
    ImGuiPlatformImeData* ime_data = &g.PlatformImeData;
    if (g.PlatformIO.Platform_SetImeDataFn != NULL && memcmp(ime_data, &g.PlatformImeDataPrev, sizeof(ImGuiPlatformImeData)) != 0)
    {
        IMGUI_DEBUG_LOG_IO("[io] Calling Platform_SetImeDataFn(): WantVisible: %d, InputPos (%.2f,%.2f)\n", ime_data->WantVisible, ime_data->InputPos.x, ime_data->InputPos.y);
        IM_ASSERT(ime_data->ViewportId == IMGUI_VIEWPORT_DEFAULT_ID); // master branch
        ImGuiViewport* viewport = GetMainViewport();
        g.PlatformIO.Platform_SetImeDataFn(&g, viewport, ime_data);
    }
    g.WantTextInputNextFrame = ime_data->WantTextInput ? 1 : 0;

I think I'd prefer the backend interface in Python to use this standard approach instead of sneakily exposing some members of the context struct.

Bear with me, I think this feature can be added, but I want to be in alignment with how Imgui expects it to be done.

[KarryCharon]

SetImeDataFn

Yes, you're right! haha