Add ARGBExtractAlpha function

source/row_common.cc

 /*
  *  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 2363 matching lines @@
     dst[3] = src[3];
     dst[7] = src[7];
     dst += 8;
     src += 8;
   }
   if (width & 1) {
     dst[3] = src[3];
   }
 }
 
+void ARGBExtractAlphaRow_C(const uint8* src_argb, uint8* dst_a, int width) {
+  int i;
+  for (i = 0; i < width - 1; i += 2) {
+    dst_a[0] = src_argb[3];
+    dst_a[1] = src_argb[7];
+    dst_a += 2;
+    src_argb += 8;
+  }
+  if (width & 1) {
+    dst_a[0] = src_argb[3];
+  }
+}
+
 void ARGBCopyYToAlphaRow_C(const uint8* src, uint8* dst, int width) {
   int i;
   for (i = 0; i < width - 1; i += 2) {
     dst[3] = src[0];
     dst[7] = src[1];
     dst += 8;
     src += 2;
   }
   if (width & 1) {
     dst[3] = src[0];
@@ skipping 211 matching lines @@
     dst_rgb565 += twidth * 2;
     width -= twidth;
   }
 }
 #endif
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif