[Feature Request] Allow display-mode selection when utilizing external screens.

[isaboll1]

From what I understand, SteamOS currently through gamescope utilizes the maximum/native display mode that is supported from the monitor. While in many cases this seems to be favored, in cases where a monitor doesn't support HDMI 2.0, yet is made as a 4k monitor this can cause issues, such as lower FPS in games than expected (as the maximum refresh rate for a 4k monitor without HDMI 2.0 is 30Hz, making the display mode cap out at 3840x2160 @30hz, which is what's then used).

From what I currently understand as well, Gamescope seems to have internal functions for setting the physical display-mode properties through it's "drm_set_mode" function.

I am requesting for the physical display's display-mode properties to be exposed and selectable from the SteamOS client, specifically when the Steam Deck or enabled devices with SteamOS are connected to external displays. I believe this would help address the issue that I mentioned above with respect to using a SteamOS enabled device, or the Steam Deck connected to a 4k screen that has no support for HDMI 2.0, as well as any other potentially similar issues with respect to monitor display-modes. The issue mentioned above would be addressed with this, as the user could select a display-mode that has a more acceptable refresh rate (such at 1920x1080@60Hz, or 2160x1440@60Hz, if the monitor supports those).