avcodec/svq1enc: output ident string in extradata field

This will enable the acurate identification of FFmpeg produced
SVQ1 streams, should there be new bugs found in the encoder.

libavcodec/svq1enc.c

@@ -41,6 +41,7 @@
 #include "svq1.h"
 #include "svq1encdsp.h"
 #include "svq1enc_cb.h"
+#include "version.h"
 
 #include "libavutil/avassert.h"
 #include "libavutil/frame.h"
@@ -572,6 +573,19 @@ static av_cold int svq1_encode_end(AVCodecContext *avctx)
     return 0;
 }
 
+static av_cold int write_ident(AVCodecContext *avctx, const char *ident)
+{
+    int size = strlen(ident);
+    avctx->extradata = av_malloc(size + 8);
+    if (!avctx->extradata)
+        return AVERROR(ENOMEM);
+    AV_WB32(avctx->extradata, size + 8);
+    AV_WL32(avctx->extradata + 4, MKTAG('S', 'V', 'Q', '1'));
+    memcpy(avctx->extradata + 8, ident, size);
+    avctx->extradata_size = size + 8;
+    return 0;
+}
+
 static av_cold int svq1_encode_init(AVCodecContext *avctx)
 {
     SVQ1EncContext *const s = avctx->priv_data;
@@ -632,7 +646,7 @@ static av_cold int svq1_encode_init(AVCodecContext *avctx)
 
     ff_h263_encode_init(&s->m); // mv_penalty
 
-    return 0;
+    return write_ident(avctx, s->avctx->flags & AV_CODEC_FLAG_BITEXACT ? "Lavc" : LIBAVCODEC_IDENT);
 }
 
 static int svq1_encode_frame(AVCodecContext *avctx, AVPacket *pkt,

tests/ref/vsynth/vsynth1-svq1

-78cdca850b19faf3aac0b0682207451e *tests/data/fate/vsynth1-svq1.mov
-1333541 tests/data/fate/vsynth1-svq1.mov
+89b1ec4d7bbee1ed2710c8cc8c1e269d *tests/data/fate/vsynth1-svq1.mov
+1333561 tests/data/fate/vsynth1-svq1.mov
 0b9ee47ee4bf735fe3697daad64fc409 *tests/data/fate/vsynth1-svq1.out.rawvideo
 stddev:    9.57 PSNR: 28.50 MAXDIFF:  210 bytes:  7603200/  7603200

tests/ref/vsynth/vsynth2-svq1

-42578021105a2f526179c5601e635312 *tests/data/fate/vsynth2-svq1.mov
-940337 tests/data/fate/vsynth2-svq1.mov
+14f355a06d475dcf6a90ac6ab3ae2970 *tests/data/fate/vsynth2-svq1.mov
+940357 tests/data/fate/vsynth2-svq1.mov
 ba8f6b721a8e19fe8a6ef92a8cff7479 *tests/data/fate/vsynth2-svq1.out.rawvideo
 stddev:    3.71 PSNR: 36.72 MAXDIFF:  210 bytes:  7603200/  7603200

tests/ref/vsynth/vsynth3-svq1

-03805cb764c00c2162b2bed24b7f34bd *tests/data/fate/vsynth3-svq1.mov
-40757 tests/data/fate/vsynth3-svq1.mov
+969dcdd69774b9c42dcf81e8dd393364 *tests/data/fate/vsynth3-svq1.mov
+40777 tests/data/fate/vsynth3-svq1.mov
 a99efde992a2e3efcc085ecc6920a1e3 *tests/data/fate/vsynth3-svq1.out.rawvideo
 stddev:   14.49 PSNR: 24.91 MAXDIFF:  183 bytes:    86700/    86700

tests/ref/vsynth/vsynth_lena-svq1

-7534b2c6b7fc7201f193e9b4514cdb90 *tests/data/fate/vsynth_lena-svq1.mov
-766817 tests/data/fate/vsynth_lena-svq1.mov
+8890d9ca13934391b6891ac5f67897c6 *tests/data/fate/vsynth_lena-svq1.mov
+766837 tests/data/fate/vsynth_lena-svq1.mov
 85261558fa744ef468fe77dbe4d91d8d *tests/data/fate/vsynth_lena-svq1.out.rawvideo
 stddev:    3.23 PSNR: 37.93 MAXDIFF:   61 bytes:  7603200/  7603200