new feature: path interpolation with gradients

I have implemented gradient interpolation for the path interpolation extension.

Here are some examples:

linear gradients:

radial gradients:

fill and stroke gradients with different amounts of stops can be interpolated, but we can't interpolate a linear gradient with a radial at the moment.

I added some test data but it is not used yet by the test suite. I thought I'd get some feedback on it first.

Let me know if you find it interesting and what needs to be done to get it merged in case you want to.