Move all knowledge of X sampling into SkScaledCodec

src/codec/SkSampler.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkSampler_DEFINED
+#define SkSampler_DEFINED
+
+#include "SkTypes.h"
+
+class SkSampler : public SkNoncopyable {
+public:
+    /**
+     *  Update the sampler to sample every sampleX'th pixel. Returns the
+     *  width after sampling.
+     */
+    int setSampleX(int sampleX) {
+        return this->onSetSampleX(sampleX);
+    }
+
+    virtual ~SkSampler() {}
+private:
+    virtual int onSetSampleX(int) = 0;
+};
+
+#endif // SkSampler_DEFINED