Corner Element (RDL)
Contains the items in the corner region of the matrix.
If the Corner element is omitted, no items are rendered in the corner.
The height of the corner item is the sum of the heights of the column headers. The width of the corner item is the sum of the widths of the row headers.
The ReportItems element within the Corner element can contain one and only one child item. Values for the Top, Left, Height and Width elements for this child item are ignored: Top and Left values are set to 0, and Height and Width values are set to 100 percent of the containing corner item.