Images with examples of gradient fills

[henrikt-ma]

These are the images that I promised to provide for #3789.

[HansOlsson]

I'll let someone working more on graphics look at it, @DagBruck

However, I do note that Dymola have traditionally used a different sphere-gradient for rectangles. One possibility would be to skip that case.

[henrikt-ma]

This PR only provides examples for the specification text proposed in #3789. I think we should try to have discussions about the actual specification in #3789, and only discuss the examples as such in this PR.

[HansOlsson]

I think I would prefer these images without the side-circles and triangle (intending to show the two colors), possibly having those colors somewhere else for reference. Just having a gradient between black and white instead of a copper tone may also reduce the need for reference colors.