mathutil: Add roundDownPow2. Bug: angleproject:5857 Change-Id: I6c7ae92a41957e9e6b40b6449f763a49493488d1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3236996 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>