C#/C++ interop pixel drawing

// XAML WPF code
<Grid>
    <Border Width="502" Height="502" BorderThickness="1" BorderBrush="Red">
        <Image x:Name="Img" Width="500" Height="500"/>
    </Border>
</Grid>

// C# Code

private readonly WriteableBitmap bitmap;

public MainWindow() {
    InitializeComponent();
    this.Img.Source = this.bitmap = new WriteableBitmap(500, 500, 96d, 96d, PixelFormats.Bgr24, null);
    this.Img.MouseMove += this.Img_MouseMove;
}

private void Img_MouseMove(object sender, MouseEventArgs e) {
    unsafe {
        const int w = 5, h = 5;
        Point p = e.GetPosition(this.Img);
        if (p.X <= (500 - w) && p.Y <= (500 - h)) {
            this.bitmap.Lock();
            DrawRectangleFast(this.bitmap, (int) p.X, (int) p.Y, w, h, Colors.Red);
            this.bitmap.Unlock();
        }
    }
}

public readonly struct COLOUR {
    public readonly byte r;
    public readonly byte g;
    public readonly byte b;

    public COLOUR(byte r, byte g, byte b) {
        this.r = r;
        this.g = g;
        this.b = b;
    }
}

[DllImport("../../../Debug/FrameControl.Native.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void draw_pixel(IntPtr backBuf, int bbpx, int stride, int l, int t, int w, int h, COLOUR colour);

public static void DrawRectangleFast(WriteableBitmap bitmap, int left, int top, int width, int height, Color color) {
    draw_pixel(bitmap.BackBuffer, bitmap.Format.BitsPerPixel / 8, bitmap.BackBufferStride, left, top, width, height, new COLOUR(color.R, color.G, color.B));
    bitmap.AddDirtyRect(new Int32Rect(left, top, width, height));
}

// C++ Lib Side
//   Transformed code a bit with the help of this post:
//   https://stackoverflow.com/questions/443867/drawing-pixels-in-wpf

#define EXPORT _declspec(dllexport)

#include <stdint.h>

struct COLOUR {
    uint8_t r;
    uint8_t g;
    uint8_t b;
};

extern "C" {
    EXPORT void draw_pixel(uint8_t* bbuf, int bytePerPx, int stride, int left, int top, int w, int h, COLOUR col) {
        // Compute the pixel's color
        int colorData = col.r << 16; // R
        colorData |= col.g << 8; // G
        colorData |= col.b << 0; // B
        for (int y = 0; y < h; y++) {
            // Get a pointer to the back buffer

            // int* would not work for some reason

            // this could also be type char* or int. It cannot be a non-byte pointer, because
            // the offset calculations done below this line are byte-based, meaning the
            // compiler will include the size of the type in the offset calclations.
            uint8_t* pBackBuffer = bbuf;

            // Find the address of the pixel to draw
            pBackBuffer += (top + y) * stride;
            pBackBuffer += left * bytePerPx;

            for (int x = 0; x < w; x++) {
                // Assign the color data to the pixel
                *(int*) pBackBuffer = colorData;
                // Increment the address of the pixel to draw
                pBackBuffer += bytePerPx;
            }
        }
    }
}