Go back to float precision throughout approximation code

Change-Id: Id7dc92fa6d1c916a1511931d3422622b7127c357
Reviewed-on: https://skia-review.googlesource.com/118593
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
22 files changed