Radial Gradient API improvement

HTML5 canvas element supports radial gradients, which cannot be completely implemented using the current QRadialGradient API. This is specified here:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-createradialgradient. QRadialGradient::QRadialGradient only take 5 parameters, but canvas createRadialGradient take 6 parameters.

Because of this limitation, http://waplabdc.nokia-boston.com/browser/users/ligman/canvas%20tests/colors%20and%20styles/radial_gradient.html fails on Qt Test Browser.