Option --create-groups should only factorize known attributes

[Jehan]

I had a SVG with SMIL animation which I tried to optimize with scour (actually Inkscape but it turns out they use scour in backend. Ref: https://bugs.launchpad.net/inkscape/+bug/1638040).

Many <animate*> tags had a fill attribute with freeze value and scour therefore factorized them inside a single group. The problem is that the SVG spec says nowhere that the value of fill on the parent node is used when it is absent. It only says that remove is the default value (cf. https://www.w3.org/TR/SVG11/single-page.html#animate-TimingAttributes). As a consequence the SVG animation ended broken since I was relying on pieces of animation getting frozen to their last frame.

The create-groups option should only create groups on attributes known to work with the parent's value.

[Jehan]

P.S.: tested with scour 0.35.

[Ede123]

Issue seems to have been discovered before: https://github.com/scour-project/scour/blob/210c5f64abdf336e25c58ca927093b3aa94c0a0c/scour/scour.py#L1007