Use LibraryImport for custom ImGuiNative functinos

imgui/Dalamud.Bindings.ImGui/Custom/ImGuiNative.Custom.cs

@@ -1,3 +1,40 @@
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
 namespace Dalamud.Bindings.ImGui;
 
-public static partial class ImGuiNative;
+public static unsafe partial class ImGuiNative
+{
+    private const string LibraryName = "cimgui";
+
+    static ImGuiNative()
+    {
+        if (LibraryName != ImGui.GetLibraryName())
+        {
+            throw new(
+                $"{nameof(LibraryName)}(={LibraryName})" +
+                $" does not match " +
+                $"{nameof(ImGui)}.{nameof(ImGui.GetLibraryName)}(={ImGui.GetLibraryName()})");
+        }
+    }
+
+    [LibraryImport($"{LibraryName}.dll", EntryPoint = "ImDrawList_AddCallback")]
+    [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    public static partial void AddCallback(
+        ImDrawList* self,
+        delegate*<ImDrawList*, ImDrawCmd*, void> callback,
+        void* callbackData = null);
+
+    [LibraryImport($"{LibraryName}.dll", EntryPoint = "igInputTextEx")]
+    [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    public static partial int InputTextEx(
+        byte* label,
+        byte* hint,
+        byte* buf,
+        int bufSize,
+        Vector2 sizeArg,
+        ImGuiInputTextFlags flags = ImGuiInputTextFlags.None,
+        delegate* unmanaged[Cdecl]<ImGuiInputTextCallbackData*, int> callback = null,
+        void* userData = null);
+}