Refactor texture compressors into separate files

src/utils/SkTextureCompressor_R11EAC.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkTextureCompressor_R11EAC_DEFINED
+#define SkTextureCompressor_R11EAC_DEFINED
+
+#include "SkBlitter.h"
+
+namespace SkTextureCompressor {
+
+    bool CompressA8ToR11EAC(uint8_t* dst, const uint8_t* src,
+                            int width, int height, int rowBytes);
+
+    SkBlitter* CreateR11EACBlitter(int width, int height, void* outputBuffer);
+}
+
+#endif  // SkTextureCompressor_R11EAC_DEFINED