xrandr - panning 1080p on 1600x1200

[H.Sachs]

Hallo,
i want to pan 1920x1080 on a 1600x1200 display on native/unscaled
resolution (1600x1200). The y-resolution of the panning resolution is
smaller than the display resolution. The x-resolution is larger. That
seems to be a problem.

xrandr --newmode 1920x1080 173.00  1920 2048 2248 2576  1080 1083 1088
1120 -hsync +vsync
xrandr --addmode DVI-D-1 1920x1080
xrandr --output DVI-D-1 --mode 1600x1200 --fb 1920x1200 --panning 1920x1080

X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  29 (RRSetPanning)
  Serial number of failed request:  37
  Current serial number in output stream:  37

The following commands do work but don't match the requirements:
xrandr --output DVI-D-1 --mode 1600x1200 --fb 1920x1200 --panning 1920x1200
xrandr --output DVI-D-1 --mode 1280x960 --fb 1920x1080 --panning 1920x1080

How can i enable a "letterbox" mode (with 120px black area) so i still
get pixel exact (native) resolution (1600x1200) for a panning 1920x1080
resolution?

ArchLinux 4.15.9
X Server 1.19.6
xrandr 1.5.0
nVidia GTX770 Kepler/NVE4/GK104 (driver: modesetting/kms)