Use raster pipeline for repeat/mirror radial gradients

(reland of https://skia-review.googlesource.com/c/24283/)

The legacy clamp specialization is still faster, but for repeat and
mirror RP wins (at low color stop counts).

Change-Id: I97a77188e912239f31e94600021b469ce7c40797
Reviewed-on: https://skia-review.googlesource.com/24742
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed