use 64bit math to compute is a rect is empty

Will work next to try to make isEmpty() private

Bug: skia:7470
Bug:799715
Change-Id: I7b43028ecd86dca68e0c67225712516d2f2f88a2
Reviewed-on: https://skia-review.googlesource.com/92620
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
6 files changed