private void SaveSnapshot()
{
//assign bmp to picturebox image to update
PictureBox1.Image = bmp;
PictureBox1.Update();
if (_redoStack.Count > 0)
{
_redoStack.Clear();
Redo_Button.Enabled = false;
}
_undoStack.Push(new Bitmap(bmp));
Undo_Button.Enabled = true;
}