stylecontext: Compute if things changed before invalidating
This is only a small performance boost by itself, but it's necessary for animations, so we need it. Benchmark numbers for my Glade benchmark for interested people: GTK 3.4.0 last commit this commit Raleigh real 0m41.879s 0m10.176s 0m9.900s user 0m41.394s 0m9.895s 0m9.628s sys 0m0.111s 0m0.096s 0m0.102s Adwaita (*) real 0m51.049s 0m13.432s 0m14.848s 0m12.253s user 0m50.487s 0m13.034s 0m13.218s 0m11.927s sys 0m0.117s 0m0.151s 0m0.147s 0m0.107s Ambiance (patched to not use private GTK APIs) real 0m52.167s 0m13.115s 0m13.117s 0m12.944s user 0m51.576s 0m12.739s 0m12.768s 0m12.651s sys 0m0.119s 0m0.137s 0m0.136s 0m0.118s (*) Adwaita and unico currently use custom properties, and _gtk_css_value_compare() for custom properties always returns FALSE, which makes this optimization never trigger. So I modified _gtk_css_value_compare() to return TRUE for these properties instead and reran the benchmark. Those are the numbers.
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