Extend SkImageGenerator to support natively generated GrTextures

src/gpu/SkGrPriv.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 SkGrPriv_DEFINED
+#define SkGrPriv_DEFINED
+
+#include "GrTypes.h"
+#include "SkPoint.h"
+
+class GrCaps;
+class GrUniqueKey;
+
+/**
+ *  Our key includes the offset, width, and height so that bitmaps created by extractSubset()
+ *  are unique.
+ *
+ *  The imageID is in the shared namespace (see SkNextID::ImageID()
+ *      - SkBitmap/SkPixelRef
+ *      - SkImage
+ *      - SkImageGenerator
+ *
+ *  Note: width/height must fit in 16bits for this impl.
+ */
+void GrMakeKeyFromImageID(GrUniqueKey* key, uint32_t imageID,
+                          U16CPU width, U16CPU height, SkIPoint origin,
+                          const GrCaps&, SkImageUsageType);
+
+#endif