v4l2: support compressed formats

Let pass the codec name to -pixel_format and introduce -input_format.

libavdevice/v4l2.c

@@ -708,11 +708,15 @@ static int v4l2_read_header(AVFormatContext *s1, AVFormatParameters *ap)
     }
 
     if (s->pixel_format) {
+        AVCodec *codec = avcodec_find_decoder_by_name(s->pixel_format);
+
+        if (codec)
+            s1->video_codec_id = codec->id;
 
         pix_fmt = av_get_pix_fmt(s->pixel_format);
 
-        if (pix_fmt == PIX_FMT_NONE) {
-            av_log(s1, AV_LOG_ERROR, "No such pixel format: %s.\n",
+        if (pix_fmt == PIX_FMT_NONE && !codec) {
+            av_log(s1, AV_LOG_ERROR, "No such input format: %s.\n",
                    s->pixel_format);
 
             res = AVERROR(EINVAL);
@@ -818,7 +822,8 @@ static const AVOption options[] = {
     { "standard",     "TV standard, used only by analog frame grabber",            OFFSET(standard),     AV_OPT_TYPE_STRING, {.str = NULL }, 0, 0,       DEC },
     { "channel",      "TV channel, used only by frame grabber",                    OFFSET(channel),      AV_OPT_TYPE_INT,    {.dbl = 0 },    0, INT_MAX, DEC },
     { "video_size",   "A string describing frame size, such as 640x480 or hd720.", OFFSET(video_size),   AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       DEC },
-    { "pixel_format", "",                                                          OFFSET(pixel_format), AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       DEC },
+    { "pixel_format", "Preferred pixel format",                                    OFFSET(pixel_format), AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       DEC },
+    { "input_format", "Preferred pixel format (for raw video) or codec name",      OFFSET(pixel_format), AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       DEC },
     { "framerate",    "",                                                          OFFSET(framerate),    AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       DEC },
     { "list_formats", "List available formats and exit",                           OFFSET(list_format),  AV_OPT_TYPE_INT,    {.dbl = 0 },  0, INT_MAX, DEC, "list_formats" },
     { "all",          "Show all available formats",                                OFFSET(list_format),  AV_OPT_TYPE_CONST,  {.dbl = V4L_ALLFORMATS  },    0, INT_MAX, DEC, "list_formats" },