PhoshOverview: Make sure the requested thumbnail is scaled according to widget size

The last-minute change before merging !481 (merged) introduced a performance regression due to accidentally disabling compositor-side thumbnail scaling. This change brings it back, which should noticeably improve performance on the Librem 5.

Basically I've used the wrong widget for size-allocation and haven't realized that until I tried it on device and noticed it became slower (it was still fast on the PC)... 🤦️