avcodec/scpr: Use av_memcpy_backptr() in type 17 and 33

This makes the changed code-path faster.

Change not tested except with the fuzzer testcase as I found no other testcase.

Improves: Timeout (136sec -> 74sec)
Improves: 16040/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCPR_fuzzer-5705876062601216
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg

libavcodec/scpr.c

@@ -532,7 +532,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
                             s->current_frame->linesize[0] / 4);
     } else if (type == 17 || type == 33) {
         uint32_t clr, *dst = (uint32_t *)s->current_frame->data[0];
-        int x, y;
+        int y;
 
         frame->key_frame = 1;
         bytestream2_skip(gb, 1);
@@ -548,9 +548,8 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
             clr = bytestream2_get_le24(gb);
         }
         for (y = 0; y < avctx->height; y++) {
-            for (x = 0; x < avctx->width; x++) {
-                dst[x] = clr;
-            }
+            dst[0] = clr;
+            av_memcpy_backptr((uint8_t*)(dst+1), 4, 4*avctx->width - 4);
             dst += s->current_frame->linesize[0] / 4;
         }
     } else if (type == 0 || type == 1) {