Add ES3 versions of min/max/mix/clamp to public SkSL.

I've also added a new runtime_intrinsics slide which plots all of the
ES3 section 8.3 intrinsics for testing purposes. We already had SkSL
tests for the ES3 versions of these functions in intrinsics/.

Change-Id: Ie4cec34cacddc26be78dce1e641de8172fcefd99
Bug: skia:12022
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441578
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed