integrate some more comprehensive mask creattion code

[stesee]

addressing the repeated need to mark a region that should be tolerated based on a diff

snippet:

// Create a tolerance mask by expanding the diff regions. Diff pixels that are non-black will be grown by padding
// into broader white areas to tolerate local differences (e.g., filenames, timestamps).
private static SKBitmap CreateToleranceMaskFromDiff(SKBitmap diffMask, int padding = 3)
{
    var w = diffMask.Width;
    var h = diffMask.Height;
    var mask = new SKBitmap(w, h, SKColorType.Rgba8888, SKAlphaType.Unpremul);
    using var canvas = new SKCanvas(mask);
    canvas.Clear(SKColors.Black);

    using var paint = new SKPaint { Color = SKColors.White, IsAntialias = false };

    // For each non-black pixel in diff, draw a filled rectangle expanded by padding.
    for (int y = 0; y < h; y++)
    {
        for (int x = 0; x < w; x++)
        {
            var col = diffMask.GetPixel(x, y);
            // consider any non-transparent / non-black pixel as part of diff
            if (col.Alpha > 16 && (col.Red > 16 || col.Green > 16 || col.Blue > 16))
            {
                var left = Math.Max(0, x - padding);
                var top = Math.Max(0, y - padding);
                var right = Math.Min(w, x + padding + 1);
                var bottom = Math.Min(h, y + padding + 1);
                var rect = new SKRectI(left, top, right, bottom);
                canvas.DrawRect(rect, paint);
            }
        }
    }

    canvas.Flush();
    return mask;
}