Add updatePlatformWindows and renderPlatformWindowsDefault functions

[defaultgnome]

To make the viewport_enable ConfigFlag work we should call a updatePlatformWindows and renderPlatformWindowsDefaultper ImGui documentation.

zgui.io.setConfigFlags(.{
    .viewport_enable = true,
});

// ... later in the main loop
{ // Enable Multi-Viewports
    const ctx = glfw.getCurrentContext(); // this is need for opengl3 see https://github.com/ocornut/imgui/blob/docking/examples/example_glfw_opengl3/main.cpp
    zgui.updatePlatformWindows();
    zgui.renderPlatformWindowsDefault();
    glfw.makeContextCurrent(ctx);
}

I tested it on my project that use zopengl and zglfw.

Resolves #29

[hazeycode]

Thanks ❤️