Fix log2() accuracy. Bug 28474248 Change-Id: I61bf28687623437c6b9b383dc189ffa28ad24c22 Reviewed-on: https://swiftshader-review.googlesource.com/5330 Tested-by: Nicolas Capens <capn@google.com> Reviewed-by: Alexis Hétu <sugoi@google.com> Reviewed-by: Nicolas Capens <capn@google.com>