Image size filter is not working as expected

[Artur Chamier]

1. Environment
Linux Mint 18.2 Cinnamon 64-bit
Dell Precision 7520 (maximum screen resolution 3840x2160)
Laptop screen + 2 external, all three screens set for 1920x1080p

2. Steps to reproduce
* Preferences -> Color and size -> check 'Are big at least 80% of the screen resolution'

3. Actual problem
* images equal to 1920x1080p or smaller are not selected to display
* some bigger images like for example 3840x2160 also not selected to display (I've got many of those in my Favorites, but none of them were displayed)

4. Workaround:
* when set 'Are big at least 50% of the screen resolution' all images are selected to display, (tested on both 2160p and 1080p images)

5. Expected:
When 3 screens are set for same resolution, I would expect that "80% filter" would accept images 1536x864 (roughly 80% of 1920x1080) or larger