XRandR supports applying transformations to an output's picture
including changes to scale. Such scaling is used by some desktop
environments under feature names such as "fractional scaling" to
accomodate HiDPI devices. Alternatively, such scaling can be enabled by
a command such as the following:
xrandr --output DP1 --scale 0.5x0.5 --filter nearest
XRandR scaling has no "HiDPI awareness" or other way for an application
to signal that it wants to work with physical display pixels, and so all
we do is scale SDL's returned display modes so that applications receive
the number of usable pixels that they expect.
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