lavc/codec_desc: add a property for codecs that support field coding

Multiple places currently use AVCodecContext.ticks_per_frame > 1 to
identify such codecs, which
* requires a codec context
* requires it to be open

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+2023-05-xx - xxxxxxxxxx - lavc 60.12.100 - codec_desc.h
+  Add AV_CODEC_PROP_FIELDS.
+
 2023-05-xx - xxxxxxxxxx - lavc 60 - codec.h
   Depreate AV_CODEC_CAP_SUBFRAMES without replacement.
 

libavcodec/codec_desc.c

@@ -38,14 +38,20 @@ static const AVCodecDescriptor codec_descriptors[] = {
         .type      = AVMEDIA_TYPE_VIDEO,
         .name      = "mpeg1video",
         .long_name = NULL_IF_CONFIG_SMALL("MPEG-1 video"),
-        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER,
+        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER |
+                     // FIXME this is strigly speaking not true, as MPEG-1 does
+                     // not allow field coding, but our mpeg12 code (decoder and
+                     // parser) can sometimes change codec id at runtime, so
+                     // this is safer
+                     AV_CODEC_PROP_FIELDS,
     },
     {
         .id        = AV_CODEC_ID_MPEG2VIDEO,
         .type      = AVMEDIA_TYPE_VIDEO,
         .name      = "mpeg2video",
         .long_name = NULL_IF_CONFIG_SMALL("MPEG-2 video"),
-        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER,
+        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER |
+                     AV_CODEC_PROP_FIELDS,
         .profiles  = NULL_IF_CONFIG_SMALL(ff_mpeg2_video_profiles),
     },
     {
@@ -225,7 +231,8 @@ static const AVCodecDescriptor codec_descriptors[] = {
         .type      = AVMEDIA_TYPE_VIDEO,
         .name      = "h264",
         .long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
-        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_LOSSLESS | AV_CODEC_PROP_REORDER,
+        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_LOSSLESS |
+                     AV_CODEC_PROP_REORDER | AV_CODEC_PROP_FIELDS,
         .profiles  = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
     },
     {
@@ -529,7 +536,8 @@ static const AVCodecDescriptor codec_descriptors[] = {
         .type      = AVMEDIA_TYPE_VIDEO,
         .name      = "vc1",
         .long_name = NULL_IF_CONFIG_SMALL("SMPTE VC-1"),
-        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER,
+        .props     = AV_CODEC_PROP_LOSSY | AV_CODEC_PROP_REORDER |
+                     AV_CODEC_PROP_FIELDS,
         .profiles  = NULL_IF_CONFIG_SMALL(ff_vc1_profiles),
     },
     {

libavcodec/codec_desc.h

@@ -90,6 +90,12 @@ typedef struct AVCodecDescriptor {
  * equal.
  */
 #define AV_CODEC_PROP_REORDER       (1 << 3)
+
+/**
+ * Video codec supports separate coding of fields in interlaced frames.
+ */
+#define AV_CODEC_PROP_FIELDS        (1 << 4)
+
 /**
  * Subtitle codec is bitmap based
  * Decoded AVSubtitle data can be read from the AVSubtitleRect->pict field.

libavcodec/version.h

@@ -29,7 +29,7 @@
 
 #include "version_major.h"
 
-#define LIBAVCODEC_VERSION_MINOR  11
+#define LIBAVCODEC_VERSION_MINOR  12
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \