Incorrect gradient rendering

[mrbean-bremen]

This has been introduced with the refactoring of SvgPaintServerFactory in #564. At least two of the W3C images render incorrectly after this change:
pservers-grad-20-b.svg (fallback not used for stroke)

struct-use-05-b.svg (image not used for gradient rendering)

[H1Gdev]

@mrbean-bremen
Next, I will check this issue.

[H1Gdev]

It is difficult and time-consuming to resolve struct-use-05-b.svg so that the various problems resolved by #564 do not recur.

[mrbean-bremen]

Well, thanks for trying!