avcodec/screenpresso: Optimize sum_delta_flipped()

553 -> 332 sec

Testcase: 20280/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCREENPRESSO_fuzzer-6238663432470528
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/screenpresso.c

@@ -94,8 +94,9 @@ static void sum_delta_flipped(uint8_t       *dst, int dst_linesize,
 {
     int i;
     for (; height > 0; height--) {
+        const uint8_t *src1 = &src[(height - 1) * src_linesize];
         for (i = 0; i < bytewidth; i++)
-            dst[i] += src[(height - 1) * src_linesize + i];
+            dst[i] += src1[i];
         dst += dst_linesize;
     }
 }