collection-icon-view: Use small thumbs on small windows

If the window's width is less than half the width of a 1920×1080 screen,
display the game thumbnails at half the size to see more of them rather
than a few huge thumbnails, making Games more usable on small screens.
4 jobs for wip/aplazas/thumbnail-size in 14 minutes and 11 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed flatpak:app #64857

00:12:03

 
  Test
passed flatpak:tests #64858

00:01:17

 
  Review
passed review #64859

00:00:24

passed stop_review #64860
manual

00:00:26