Fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/306
I added a ShaderEffectSource in between so that the blur only samples the wallpaper when necessary
I also switched to FastBlur which seems to be much, much faster than MultiEffect on the PinePhone.
From testing, this made the performance about on-par with no blur at all.
The wallpaper blur causes extreme lag on the PinePhone, best to disable it for now by default, and hide it behind a setting. :C
https://invent.kde.org/plasma/plasma-mobile/-/issues/306
PS: is there a string freeze ongoing? this introduces 2 new strings