avcodec/mpeg12enc: support mpeg2 encoder const level

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

doc/encoders.texi

@@ -2902,6 +2902,16 @@ SNR Scalable
 @item simple
 @end table
 
+@item level
+Select the mpeg2 level to encode:
+
+@table @samp
+@item high
+@item high1440
+@item main
+@item low
+@end table
+
 @item seq_disp_ext @var{integer}
 Specifies if the encoder should write a sequence_display_extension to the
 output.

libavcodec/mpeg12enc.c

@@ -1167,6 +1167,12 @@ static const AVOption mpeg2_options[] = {
     {     "secam",        NULL, 0, AV_OPT_TYPE_CONST,  {.i64 = VIDEO_FORMAT_SECAM      },  0, 0, VE, "video_format" },
     {     "mac",          NULL, 0, AV_OPT_TYPE_CONST,  {.i64 = VIDEO_FORMAT_MAC        },  0, 0, VE, "video_format" },
     {     "unspecified",  NULL, 0, AV_OPT_TYPE_CONST,  {.i64 = VIDEO_FORMAT_UNSPECIFIED},  0, 0, VE, "video_format" },
+#define LEVEL(name, value) name, NULL, 0, AV_OPT_TYPE_CONST, { .i64 = value }, 0, 0, VE, "avctx.level"
+    { LEVEL("high",     4) },
+    { LEVEL("high1440", 6) },
+    { LEVEL("main",     8) },
+    { LEVEL("low",     10) },
+#undef LEVEL
     FF_MPV_COMMON_OPTS
     FF_MPEG2_PROFILE_OPTS
     { NULL },