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 {
+    // should be overridden by subclass
+    return NULL;
+}
+
+size_t SkShader::contextSize() const {
+    // should be overridden by subclass
+    return 0;
+}
+
 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