generalize xferu64 procs using proc4f

gm/xferu64.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 "gm.h"
 #include "SkCanvas.h"
 #include "SkImageInfo.h"
@@ skipping 53 matching lines @@
     SkBitmap bm;
     bm.allocPixels(info);
     SkPixmap pm;
     bm.peekPixels(&pm);
     memset(pm.writable_addr(), 0, pm.getSafeSize());
 
     if (SkColorGetA(c) == 0xFF) {
         u64_flags |= SkXfermode::kSrcIsOpaque_PM4fFlag;
     }
 
-    const SkXfermode::U64State state { nullptr, u64_flags };
+    SkXfermode::Mode mode = SkXfermode::kSrcOver_Mode;
+    SkAutoTUnref<SkXfermode> xfer(SkXfermode::Create(mode));
+
+    const SkXfermode::U64State state { xfer, u64_flags };
 
     const SkPM4f src = SkColor4f::FromColor(c).premul();
-    auto proc1 = SkXfermode::GetU64Proc1(SkXfermode::kSrcOver_Mode, u64_flags);
+    auto proc1 = SkXfermode::GetU64Proc1(mode, u64_flags);
     for (int y = 0; y < ir.height()/2; ++y) {
         proc1(state, pm.writable_addr64(0, y), src, ir.width(), aa);
     }
     
     SkPM4f buffer[1000];
     for (int i = 0; i < ir.width(); ++i) {
         buffer[i] = src;
     }
-    auto procN = SkXfermode::GetU64ProcN(SkXfermode::kSrcOver_Mode, u64_flags);
+    auto procN = SkXfermode::GetU64ProcN(mode, u64_flags);
     for (int y = ir.height()/2 + 1; y < ir.height(); ++y) {
         procN(state, pm.writable_addr64(0, y), buffer, ir.width(), aa);
     }
 
     SkAutoTUnref<SkImage> image(new_u64_image(bm, u64_flags));
     canvas->drawImage(image, r.left(), r.top(), nullptr);
 }
 
 /*
  *  Test SkXfer4fProcs directly for src-over, comparing them to current SkColor blits.
@@ skipping 43 matching lines @@
                 }
                 canvas->translate(W + 20, 0);
             }
             canvas->restore();
             canvas->translate(0, H + 20);
         }
         canvas->restore();
         canvas->translate(0, (H + 20) * SK_ARRAY_COUNT(flags) + 20);
     }
 }