OpenRaster: default groups to non-passthrough mode, per spec

This should be backported to whatever versions are still supported.

Per the OpenRaster spec: https://www.openraster.org/baseline/layer-stack-spec.html#isolation-attribute

Default value
    isolate

Which corresponds to non-passthrough mode.

The legal values are "isolate" and "auto", and because the default is specified as "isolate", the passthrough flag should be false unless the isolation value is specifically "auto".