generalize xferu64 procs using proc4f

gm/xferu64.cpp

Index: gm/xferu64.cpp
diff --git a/gm/xferu64.cpp b/gm/xferu64.cpp
index 03fc8864b56876eac812082e246f54e3bb6c0bb4..a72735f96d7ef7739429fa0885155983afe1d5be 100644
--- a/gm/xferu64.cpp
+++ b/gm/xferu64.cpp
@@ -71,10 +71,13 @@ static void draw_rect(SkCanvas* canvas, const SkRect& r, SkColor c, uint32_t u64
         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);
     }
@@ -83,7 +86,7 @@ static void draw_rect(SkCanvas* canvas, const SkRect& r, SkColor c, uint32_t u64
     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);
     }