Add ARGBExtractAlpha function

source/row_common.cc

Index: source/row_common.cc
diff --git a/source/row_common.cc b/source/row_common.cc
index 0c47e1016bf15c505e46e7bb5cbb578d8f9a120b..32d2f686f20ade444f9ee62d3b2483e946bd92e2 100644
--- a/source/row_common.cc
+++ b/source/row_common.cc
@@ -2381,6 +2381,19 @@ void ARGBCopyAlphaRow_C(const uint8* src, uint8* dst, int width) {
   }
 }
 
+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) {