used to avoid dual update traversals of the scene, which is not
allowed in my application (but I forgot the reason ;-) Now the blank
problem will disappear.
Inactive mode is also available, using the --inactive mode and
--camera-eye and --camera-hpr to set camera position:
./osgposter --output-poster --poster output.bmp --tilesize 800 600
--finalsize 8000 6000 cow.osg --inactive --camera-eye 0 0 20
"
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