Fix rounding for values near 1/2 and > 2^24

Bug: chromium:1366345, oss-fuzz:51634, oss-fuzz:51629
Change-Id: I721d3512b28b8963ec6224cabf28bc647b6ad956
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/583583
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed