SteamOS 3.8/3.9 scaling filter not applied correctly when game resolution is greater than display resolution

Your system information

• Steam client version: 1787790984
• SteamOS version: 3.8.25/3.9.0
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: Yes
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

With SteamOS 3.7.25 using a game resolution greater than display resolution together with the linear or FSR scaling filter can be used as a form of supersampling anti aliasing. On SteamOS 3.8.25 no matter which scaling filter is selected is seems the output is always scaled with a nearest neighbor filter, negating any visual improvement over rendering at native resolution. I also noticed that even on SteamOS 3.7.25 the correct filter isn't always applied either, on screenshots or when a hardware mouse cursor is overlayed the image is only scaled with a nearest neighbor filter as well.

Steps for reproducing this issue:

1. In game properties set "Maximum Game Resolution" to value greater than display resolution and enable "Allow resolution for internal display."
2. Select the resolution set above in-game
3. Make sure "Scaling Filter" is set to "Linear"

No source file or test is identified in the report. First reproduce the issue using the listed resolution and scaling-filter steps on SteamOS 3.8.25/3.9.0, compare with 3.7.25, and verify that the selected filter is applied both during display and in screenshots or with a hardware cursor.