background-paintable: Clip light wallpapers as well

Alice Mikhaylenko requested to merge wip/exalm/appearance-thumbnails into master

Turns out drawing the whole light wallpaper and then half of the dark wallpaper on top of it is leading to a case of gtk#3840 (closed)

Draw half of each instead, so they don't overlap.

Screenshot_from_2022-02-10_05-17-37 Screenshot_from_2022-02-10_05-18-06

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