Need to handle invalid Rectangle radius gracefully.

The attached screenshot was produced with the following QML:

import Qt 4.6

Rectangle {
     width: 500
     height: 500
     Rectangle {
         x: 100
         y: 100
         width: 100
         height: 100
         color: "red"
         border.width: 1
         radius: 90
     }
}

In this case, we should just draw the corner bits, and only draw that portion of the corner bits that produce a 'nice-looking' image (in this case, 4 50x50 'cropped' corners, producing a rounded diamond shape).

Ideally this is handled automatically by qDrawBroderPixmap.