SkRasterPipeline: simplify impl and remove need to rewire stages

tests/SkRasterPipelineTest.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Test.h"
 #include "SkRasterPipeline.h"
 
@@ skipping 67 matching lines @@
     p.append(store, store_tail, dst_vals);
 
     p.run(5);
 
     REPORTER_ASSERT(r, dst_vals[0] ==  1);
     REPORTER_ASSERT(r, dst_vals[1] ==  4);
     REPORTER_ASSERT(r, dst_vals[2] ==  9);
     REPORTER_ASSERT(r, dst_vals[3] == 16);
     REPORTER_ASSERT(r, dst_vals[4] == 25);
 }
+
+DEF_TEST(SkRasterPipeline_empty, r) {
+    // No asserts... just a test that this is safe to run.
+    SkRasterPipeline p;
+    p.run(20);
+}
+
+DEF_TEST(SkRasterPipeline_nonsense, r) {
+    // No asserts... just a test that this is safe to run and terminates.
+    // square() always calls st->next(); this makes sure we've always got something there to call.
+    SkRasterPipeline p;
+    p.append(square);
+    p.run(20);
+}