vp8: modifcatiion to skin map computation.

For each block in pickinter: use average of four middle
pixels (instead of single pixel) to set skin map.
This can help a little in reducing false skin detection in
some cases.

Change-Id: Ic247af75e9c2948b08ab977a39e061adacd8ec97

vp8/encoder/pickinter.c

diff --git a/vp8/encoder/pickinter.c b/vp8/encoder/pickinter.c
index f3443db..4ec9c05 100644
--- a/vp8/encoder/pickinter.c
+++ b/vp8/encoder/pickinter.c
@@ -813,9 +813,18 @@
 
     // Check if current macroblock is in skin area.
     {
-    const int y = x->src.y_buffer[7 * x->src.y_stride + 7];
-    const int cb = x->src.u_buffer[3 * x->src.uv_stride + 3];
-    const int cr = x->src.v_buffer[3 * x->src.uv_stride + 3];
+    const int y = (x->src.y_buffer[7 * x->src.y_stride + 7] +
+        x->src.y_buffer[7 * x->src.y_stride + 8] +
+        x->src.y_buffer[8 * x->src.y_stride + 7] +
+        x->src.y_buffer[8 * x->src.y_stride + 8]) >> 2;
+    const int cb = (x->src.u_buffer[3 * x->src.uv_stride + 3] +
+        x->src.u_buffer[3 * x->src.uv_stride + 4] +
+        x->src.u_buffer[4 * x->src.uv_stride + 3] +
+        x->src.u_buffer[4 * x->src.uv_stride + 4]) >> 2;
+    const int cr = (x->src.v_buffer[3 * x->src.uv_stride + 3] +
+        x->src.v_buffer[3 * x->src.uv_stride + 4] +
+        x->src.v_buffer[4 * x->src.uv_stride + 3] +
+        x->src.v_buffer[4 * x->src.uv_stride + 4]) >> 2;
     x->is_skin = 0;
     if (!cpi->oxcf.screen_content_mode)
       x->is_skin = is_skin_color(y, cb, cr);