SkRasterPipeline: add last() and docs.

Today if you use the simple SK_RASTER_STAGE interface to build a pipeline, each stage you add calls into a next stage.  The last stage you add calls into a special backstop stage JustReturn that, well, just returns, ending the pipeline.

This adds last(), which cuts that last stage off the pipeline.  Instead, the stage you add using last() returns directly, ending the pipeline itself without jumping into JustReturn.

This reduces the overhead of using the pipelined version of SkRasterPipelineBench from ~25% to ~20% on my desktop.

Also, add docs.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2713

Change-Id: I11469378e2765c6e34db52eb3eef648d6612da3f
Reviewed-on: https://skia-review.googlesource.com/2713
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/bench/SkRasterPipelineBench.cpp b/bench/SkRasterPipelineBench.cpp
index a5263d770f..5ef64ce603 100644
--- a/bench/SkRasterPipelineBench.cpp
+++ b/bench/SkRasterPipelineBench.cpp
@@ -174,7 +174,7 @@ public:
         p.append<   scale_u8,    scale_u8_tail>(mask);
         p.append<load_d_srgb, load_d_srgb_tail>( dst);
         p.append<srcover>();
-        p.append< store_srgb,  store_srgb_tail>( dst);
+        p.last< store_srgb,  store_srgb_tail>( dst);
 
         p.run(N);
     }