use double to compute root to avoid overflow

Bug: 850350
Change-Id: Iac04fc62e69f51b68c5fc7f55ac1be930133cc74
Reviewed-on: https://skia-review.googlesource.com/136597
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed