Extract 4f gradient interval functionality

... into structures usable outside Sk4fGradient classes.

Change-Id: Ifffdbe8bafa4f027f2016ce71eefede6034dd3ae
Reviewed-on: https://skia-review.googlesource.com/10060
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed