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And add tests to demonstrate accuracy. This is taken from https://martin.ankerl.com/2007/10/04/optimized-pow-approximation-for-java-and-c-c/ and results in a maximum error of 3% over our input range. Change-Id: I1568dabb552d52a3eff085fd327ed3c1c246edcc Reviewed-on: https://gerrit.libreoffice.org/c/core/+/184623 Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk> Tested-by: Jenkins