Add ImDrawCmd.UserCallback support

[Soreepeong]

Added to IImGuiRenderer:

• ResetDrawCmdUserCallback
• AddDrawCmdUserCallback
• RemoveDrawCmdUserCallback

Each ImDrawList.AddCallback will cause the current render parameters to be changed for the following operation. ResetDrawCmdUserCallback is provided to reset reset render parameters to the renderer defaults.

Example usage:

DeviceContext deviceContext = (valid_value);
RawDX11Scene scene = (valid_value);
PixelShader pixelShader = (valid_value);
SamplerState samplerState = (valid_value);
nint myCallback;

void Initialize() => myCallback = scene.Renderer.AddDrawCmdUserCallback(MyCallbackBody);

void Uninitialize() => scene.Renderer.RemoveDrawCmdUserCallback(MyCallbackBody);

void MyCallbackBody(ImDrawDataPtr drawData, ImDrawCmdPtr drawCmd) {
    deviceContext.PixelShader.Set(pixelShader);
    deviceContext.PixelShader.SetSampler(0, samplerState);
}

void Draw() {
    ImGui.AddCallback(myCallback, nint.Zero);
    ImGui.Image(someImage, new(1024, 1024));
    ImGui.AddCallback(scene.Renderer.ResetDrawCmdUserCallback, nint.Zero);
}

[Soreepeong]

Testing some more use cases.