effects/blendchanges: ignore fullscreen windows

Fullscreen windows are most likely games or videos, where the blend effect is an annoyance that freezes the screen for a moment when a accent color change is triggered in the background.

This is the lowest-effort solution that fixes the problem I have. A proper more long term solution could for example disable the effect with slideshow updates (which I'd personally like to have while not playing a game though), use content-type (low adoption atm) or check how often the window got updated recently (high update rate -> skip the effect).