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As discussed above, the envisaged use case for this device is to detect whether page contents require colour or not to allow printers to decide whether to rasterise for colour inks or a faster/cheaper greyscale pass.
Such printers will normally be operating in banded mode, which requires (or at least greatly benefits from) the use of a display list. By using the device in passthrough mode, the testing can be performed at the same time as the list is built.
Simply create the display list device as you would normally, and pass it into fz_new_test_device as passthrough. Then run the page contents through the returned test device. The test device will pass each call through to the underlying list device and so the display list be built as normal.
When run in this mode, the device can no longer use the `early-exit' optimisation of throwing a FZ_ABORT error.