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When the new GENERATE_REFERENCE symbol is defined to 1, no threads are involved
at all. In this case, all operations are done sequentially by the main thread.

Thus, in this mode no thread issues can occur and the result must be usable as
reference images.
Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>