Improve negotiation

Try to link ports based on compatible formats
Add methods to filter formats.

spa/include/spa/audio/format.h

@@ -55,6 +55,8 @@ SpaResult   spa_format_audio_init        (SpaMediaType     type,
                                           SpaFormatAudio  *format);
 SpaResult   spa_format_audio_parse       (const SpaFormat *format,
                                           SpaFormatAudio  *aformat);
+SpaResult   spa_format_audio_filter      (SpaFormatAudio  *format,
+                                          const SpaFormat *filter);
 
 #ifdef __cplusplus
 }  /* extern "C" */

spa/include/spa/node.h

@@ -328,7 +328,9 @@ struct _SpaNode {
    * @index: an index variable, 0 to get the first item
    *
    * Enumerate all possible formats on @port_id of @node that are compatible
-   * with @filter..
+   * with @filter. When @port_id is #SPA_ID_INVALID, the enumeration will
+   * list all the formats possible on a port that would be added with
+   * add_port().
    *
    * Use @index to retrieve the formats one by one until the function
    * returns #SPA_RESULT_ENUM_END.

spa/include/spa/video/format.h

@@ -62,6 +62,9 @@ SpaResult   spa_format_video_init       (SpaMediaType     type,
 SpaResult   spa_format_video_parse      (const SpaFormat  *format,
                                          SpaFormatVideo   *dest);
 
+SpaResult   spa_format_video_filter      (SpaFormatVideo  *format,
+                                          const SpaFormat *filter);
+
 struct _SpaFormatVideo {
   SpaFormat format;
   union {