Fix a couple float-cast-overflow in SkScan*.

Bug: skia:5060
Change-Id: I60a48993c77631aaad9354bb86b13204dc618bf4
Reviewed-on: https://skia-review.googlesource.com/47422
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Mike Reed <reed@google.com>
7 files changed