avcodec/gdv: Eliminate 50% of the reads in the first inner loop in rescale()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/gdv.c

@@ -85,9 +85,15 @@ static void rescale(GDVContext *gdv, uint8_t *dst, int w, int h, int scale_v, in
             int y = h - j - 1;
             uint8_t *dst1 = dst + PREAMBLE_SIZE + y * w;
             uint8_t *src1 = dst + PREAMBLE_SIZE + (y>>!!gdv->scale_h) * (w>>1);
-            for (x = w - 1; x >= 0; x--) {
+
+            for (x = w - 1; x >= 0 && !(x&1); x--) {
                 dst1[x] = src1[(x>>1)];
             }
+
+            for (x--; x >= 0; x-=2) {
+                dst1[x  ] =
+                dst1[x+1] = src1[(x>>1)];
+            }
         }
     } else if (gdv->scale_h) {
         for (j = 0; j < h; j++) {