Relax limits on corner radii in Rectangle

The rectangles corner radii are limited to half the width and half the height. This made sense when all radii had to be the same before https://codereview.qt-project.org/c/qt/qtdeclarative/+/477301.

However, since radii can be set individually, we can relax this limit to something like

upper-left radius + upper-right radius < width
upper-left radius + lower-left radius < height
and so on...

This increases the flexibility of rectangle substantially without increasing the code complexity.