Add ARGBExtractAlpha function

include/libyuv/planar_functions.h

 /*
  *  Copyright 2011 The LibYuv Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS. All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 270 matching lines @@
 int ARGBCopy(const uint8* src_argb, int src_stride_argb,
              uint8* dst_argb, int dst_stride_argb,
              int width, int height);
 
 // Copy Alpha channel of ARGB to alpha of ARGB.
 LIBYUV_API
 int ARGBCopyAlpha(const uint8* src_argb, int src_stride_argb,
                   uint8* dst_argb, int dst_stride_argb,
                   int width, int height);
 
+// Extract the alpha channel from ARGB.
+LIBYUV_API
+int ARGBExtractAlpha(const uint8* src_argb, int src_stride_argb,
+                     uint8* dst_a, int dst_stride_a,
+                     int width, int height);
+
 // Copy Y channel to Alpha of ARGB.
 LIBYUV_API
 int ARGBCopyYToAlpha(const uint8* src_y, int src_stride_y,
                      uint8* dst_argb, int dst_stride_argb,
                      int width, int height);
 
 typedef void (*ARGBBlendRow)(const uint8* src_argb0, const uint8* src_argb1,
                              uint8* dst_argb, int width);
 
 // Get function to Alpha Blend ARGB pixels and store to destination.
@@ skipping 191 matching lines @@
 int ARGBSobelXY(const uint8* src_argb, int src_stride_argb,
                 uint8* dst_argb, int dst_stride_argb,
                 int width, int height);
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif
 
 #endif  // INCLUDE_LIBYUV_PLANAR_FUNCTIONS_H_  NOLINT