[FEATURE] BeginTabItem missing possibility to set Flags without open/close ref bool.

[CaptainTimberTim]

Hi,
I just had the problem that the BeginTabItem cannot be called easily with flags enabled, if you don't want the open/close tab feature.
In the C version of ImGui this can be done by passing a nullpointer as the p_open param. But as C# does not allow this in normal code (as far as I know?), there would need to be an additional overload without the "ref bool p_open" param.

For me I now solved this by copying the generated implementation of the imguiNET and changing it by just passing null.

public static bool BeginTabItem(string label, ImGuiTabItemFlags flags)
{
    byte* native_label;
    int label_byteCount = 0;
    if (label != null)
    {
        label_byteCount = Encoding.UTF8.GetByteCount(label);
        if (label_byteCount > 2048)
        {
            native_label = (byte*)Marshal.AllocHGlobal(label_byteCount + 1);
        }
        else
        {
            byte* native_label_stackBytes = stackalloc byte[label_byteCount + 1];
            native_label = native_label_stackBytes;
        }
        int native_label_offset = GetUtf8(label, native_label, label_byteCount);
        native_label[native_label_offset] = 0;
    }
    else { native_label = null; }
    byte ret = ImGuiNative.igBeginTabItem(native_label, null, flags);
    if (label_byteCount > 2048)
    {
        Marshal.FreeHGlobal((IntPtr)native_label);
    }
    return ret != 0;
}

private static int GetUtf8(string s, byte* utf8Bytes, int utf8ByteCount)
{
    fixed (char* utf16Ptr = s)
    {
        return Encoding.UTF8.GetBytes(utf16Ptr, s.Length, utf8Bytes, utf8ByteCount);
    }
}

[psydack]

Hello there.

This is awesome.

If you want to make a merge request in our custom ImGui.Net project , I would really appreciate.

Edit: I will use that to remember to change the code generator.

Thank you.