added SkSL saturate() function

Bug: skia:8220
Change-Id: Ib2e58ac77345a2aa53302c6c1484d52533556f93
Reviewed-on: https://skia-review.googlesource.com/145371
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
24 files changed