DataGridView自定義RichTextBox列

https://www.codeproject.com/Articles/31823/RichTextBox-Cell-in-a-DataGridView-2

RichText是用圖片顯示的，當SelectionMode爲FullRowSelect，圖片背景顏色設置有bug。修改Paint方法解決：

        protected override void Paint(Graphics graphics, Rectangle clipBounds, Rectangle cellBounds, int rowIndex, DataGridViewElementStates cellState, object value, object formattedValue, string errorText, DataGridViewCellStyle cellStyle, DataGridViewAdvancedBorderStyle advancedBorderStyle, DataGridViewPaintParts paintParts)
        {
            base.Paint(graphics, clipBounds, cellBounds, rowIndex, cellState, null, null, errorText, cellStyle, advancedBorderStyle, paintParts);

            bool selected = base.Selected;
            if ((cellState | DataGridViewElementStates.Selected) == cellState)
            {
                selected = true;
            }

            Image img = GetRtfImage(rowIndex, value, selected);//base.Selected

            if (img != null)
                graphics.DrawImage(img, cellBounds.Left, cellBounds.Top);
        }