Add avfilter link (#935)

The avfilter link can do anything the avfilter filter can do.

Additionally, the link properly handles the adeclick filter by
providing future frames to overcome the delay required by that
avfilter.

In the future, more avfilter types can be optimized with future
frames if they also require a delay.

src/modules/avformat/CMakeLists.txt

@@ -37,7 +37,7 @@ if(TARGET PkgConfig::libavcodec)
 endif()
 
 if(TARGET PkgConfig::libavfilter)
-  target_sources(mltavformat PRIVATE filter_avfilter.c link_avdeinterlace.c)
+  target_sources(mltavformat PRIVATE filter_avfilter.c link_avdeinterlace.c link_avfilter.c)
   target_link_libraries(mltavformat PRIVATE PkgConfig::libavfilter)
   target_compile_definitions(mltavformat PRIVATE AVFILTER)
 endif()

src/modules/avformat/factory.c

@@ -32,6 +32,7 @@ extern mlt_filter filter_swscale_init(mlt_profile profile, char *arg);
 extern mlt_producer producer_avformat_init(mlt_profile profile, const char *service, char *file);
 extern mlt_filter filter_avfilter_init(mlt_profile, mlt_service_type, const char *, char *);
 extern mlt_link link_avdeinterlace_init(mlt_profile, mlt_service_type, const char *, char *);
+extern mlt_link link_avfilter_init(mlt_profile, mlt_service_type, const char *, char *);
 extern mlt_link link_swresample_init(mlt_profile profile, mlt_service_type, const char *, char *);
 
 // ffmpeg Header files
@@ -345,7 +346,11 @@ static mlt_properties avfilter_metadata(mlt_service_type type, const char *id, v
         "numeric properties have type string because they accept an expression (see FFmpeg "
         "documentation) even though they evaluate to a numeric value.");
     mlt_properties_set(metadata, "creator", "libavfilter maintainers");
-    mlt_properties_set(metadata, "type", "filter");
+    if (type == mlt_service_filter_type) {
+        mlt_properties_set(metadata, "type", "filter");
+    } else {
+        mlt_properties_set(metadata, "type", "link");
+    }
 
     mlt_properties tags = mlt_properties_new();
     mlt_properties_set_data(metadata, "tags", tags, 0, (mlt_destructor) mlt_properties_close, NULL);
@@ -506,6 +511,11 @@ MLT_REPOSITORY
                                   service_name,
                                   avfilter_metadata,
                                   (void *) f->name);
+            MLT_REGISTER(mlt_service_link_type, service_name, link_avfilter_init);
+            MLT_REGISTER_METADATA(mlt_service_link_type,
+                                  service_name,
+                                  avfilter_metadata,
+                                  (void *) f->name);
         }
     }
     mlt_properties_close(blacklist);