SVG clip intersections and operators

[janpe2]

Fixes #8496

When a new clipping path is set, the result should be the intersection of the new path with the previous clipping path. Currently the SVG backend incorrectly just replaces the previous clipping path with the new one.

SVG can automatically compute the intersection. It's easiest to add a clip-path attribute to a clipPath element. The SVG Recommendation says:

If a valid 'clip-path' reference is placed on a 'clipPath' element, the resulting clipping path is the intersection of the contents of the 'clipPath' element with the referenced clipping path.

In issue #8496 a text-shaped clipping path is first set and then a larger rectangular path. This is repeated in a number of texts on the page.
issue8496_reduced.pdf
The reduced PDF should show a black letter 'a', not a rectangle.

I have also fixed all the painting operators S s f f* B B* b b* n that should generate clipping paths. Previously only n has worked. Here is a file to test these:
clip_operators.pdf

[timvandermeij]

/botio-linux preview

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[timvandermeij]

Moreover, this fixes the majority of #8682 (only some text/images are missing, but all clipping is now correct).

[timvandermeij]

Good find; thank you for improving this!