* On X11, make `WINIT_HIDPI_FACTOR` dominate `Xft.dpi` in some cases
This commit makes `WINIT_HIDPI_FACTOR` dominate `Xft.dpi` in general and
adds a special value `0` for `WINIT_HIDPI_FACTOR` to use winit computed
DPI factor with randr over Xft.dpi.
* Use `randr` instead of `0` for auto dpi scaling
* Update CHANGELOG
* blow up on wrong env var
* Allow empty string for env var