Allow setting internal screen overscan value

[cheater]

People who install the DeckHD aftermarket screen report that the screen's bezel is a little too generous, and obscures the outermost few columns and rows of the screen. The amount of pixels on each side is different from screen to screen. For this reason, please allow setting overscan, or advise how this can be done in SteamOS. Command line usage is fine, but hopefully this is a setting one can set once, instead of on every boot.

There is relevant conversation about xrandr and overscan here:

https://www.linuxquestions.org/questions/linux-newbie-8/tv-overscan-4175704176/

but I don't know how that applies to SteamOS and the Steam BPM settings for game resolutions.

The desired result would be that games would not only see the resolution 1920x1200 (and 960x600) but would also see e.g. 1915x1194 (5 less vertical, 6 less horizontal pixels) as well as 957x597 (quarter resolution, rounded down on half-pixel amounts 975.5 -> 975) or whatever the resulting values would be depending on the overscan values decided by a user. The option to use the full physical panel including the overscan area (by setting the resolution 1920x1200) should remain, for games that become buggy when trying to run them on unusual resolutions like that.

While this is not an issue with Valve hardware, it would be greatly appreciated. Ultimately SteamOS is trying to have compatibility with non Steam Deck hardware.