add default impl for context methods on shader

src/core/SkShader.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapProcShader.h"
 #include "SkReadBuffer.h"
 #include "SkMallocPixelRef.h"
@@ skipping 45 matching lines @@
     return m->invert(totalInverse);
 }
 
 bool SkShader::validContext(const SkBitmap& device,
                             const SkPaint& paint,
                             const SkMatrix& matrix,
                             SkMatrix* totalInverse) const {
     return this->computeTotalInverse(matrix, totalInverse);
 }
 
+SkShader::Context* SkShader::createContext(const SkBitmap&, const SkPaint&, const SkMatrix&,
+                                           void* storage) const {
+    return NULL;
+}
+
+size_t SkShader::contextSize() const {
+    return 0;

[scroggo, 2014/04/30 21:38:23]

This may assert in skia/ext/pixel_ref_utils_unittest.cc after
https://codereview.chromium.org/264463003/

[Dominik Grewe, 2014/05/01 09:18:43]

Yes, this will fail if a subclass doesn't overwrite this method. In [1] we'd
pass 0 as the allocation size to SkSmallAllocator which requires that the
allocation size is at least the size of T (= SkShader::Context).
Could we return 'sizeof(SkShader::Context)' instead? Otherwise we'd have to
check for 'contextSize() == 0' before trying to allocate.

[1]
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

+}
+
 SkShader::Context::Context(const SkShader& shader, const SkBitmap& device,
                            const SkPaint& paint, const SkMatrix& matrix)
     : fShader(shader)
 {
     SkASSERT(fShader.validContext(device, paint, matrix));
 
     // Because the context parameters must be valid at this point, we know that the matrix is
     // invertible.
     SkAssertResult(fShader.computeTotalInverse(matrix, &fTotalInverse));
     fTotalInverseClass = (uint8_t)ComputeMatrixClass(fTotalInverse);
@@ skipping 254 matching lines @@
 #include "SkEmptyShader.h"
 
 void SkEmptyShader::toString(SkString* str) const {
     str->append("SkEmptyShader: (");
 
     this->INHERITED::toString(str);
 
     str->append(")");
 }
 #endif