Fix fast crop for transparent objects (cherry-pick)

feComposite Copy is not used as Inkscape can still be compiled with it disabled.

feComposite arithmetic involves us doing calculations instead of Cairo, so is roughly equivalent to feColorMatrix (no testing done), and saturate was used to allow for a short values string (defaults not interpreted correctly in order Inkscape versions)

Fixes #2419 (closed)

(cherry picked from commit 0d3cc5df)