Add dest type hint to SkShader::ContextRec

tests/SkColor4fTest.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 "SkColor.h"
 #include "SkColorMatrixFilter.h"
 #include "SkGradientShader.h"
@@ skipping 130 matching lines @@
         { make_color_sh, true },
         { make_grad_sh,  false },
         { make_image_sh, false },
         { make_cf_sh,    true },
     };
 
     SkPaint paint;
     for (const auto& rec : recs) {
         uint32_t storage[200];
         paint.setShader(rec.fFact())->unref();
-        const SkShader::ContextRec contextRec(paint, SkMatrix::I(), nullptr);
+        // Encourage 4f context selection. At some point we may need
+        // to instantiate two separate contexts for optimal 4b/4f selection.
+        const SkShader::ContextRec contextRec(paint, SkMatrix::I(), nullptr,
+                                              SkShader::ContextRec::kPM4f_DstType);
         SkASSERT(paint.getShader()->contextSize(contextRec) <= sizeof(storage));
         SkShader::Context* ctx = paint.getShader()->createContext(contextRec, storage);
         REPORTER_ASSERT(reporter, ctx->supports4f() == rec.fSupports4f);
         if (ctx->supports4f()) {
             const int N = 100;
             SkPM4f buffer4f[N];
             ctx->shadeSpan4f(0, 0, buffer4f, N);
             SkPMColor buffer4b[N];
             ctx->shadeSpan(0, 0, buffer4b, N);
             compare_spans(buffer4f, buffer4b, N, reporter);
@@ skipping 68 matching lines @@
 //
 DEF_TEST(Color4f_xfermode_proc4f, reporter) {
     // TODO: extend xfermodes so that all cases can be tested.
     //
     for (int mode = SkXfermode::kClear_Mode; mode <= SkXfermode::kScreen_Mode; ++mode) {
         SkXfermodeProc   proc32 = SkXfermode::GetProc((SkXfermode::Mode)mode);
         SkXfermodeProc4f proc4f = SkXfermode::GetProc4f((SkXfermode::Mode)mode);
         REPORTER_ASSERT(reporter, compare_procs(proc32, proc4f));
     }
 }