Big Gradients are drawn very badly with QPainter on Image render target

When painting with QPainter and using big gradients with small ranges between the colorstops, the colors in between get interpolated only very badly. The bigger the gradient, the worse the result. It does not matter, how much of the gradient is really visible.

This happens only when using Image as render target. With a FramebufferObject everything looks nice and clean.

It is the same for linear and radial gradients. A linear gradient could be changed to a smaller gradient with other colorstops though, but if you want a radial gradient with a certain radius, it is not really possible to substitute it for a smaller one.

A demo application which shows the problem is attached.