Closes #14537

- Adds server-side scale factor detection via `randr` when client-side
detection fails using `xrdb/Xft.dpi`.
- Adds the `GPUI_X11_SCALE_FACTOR` flag to force a scale factor, which
can be a positive number for custom scaling or `randr` for server-side
scale factor detection.

Release Notes:

- Fixed an issue where the scale factor was not detected correctly on
X11 systems when `Xft.dpi` is not defined (mostly in cases involving
window managers).