lavfi: add a preinit callback to filters.

It is necessary for filters with child objects, to set the class
and default options values.

libavfilter/avfilter.c

@@ -692,6 +692,7 @@ static int default_execute(AVFilterContext *ctx, avfilter_action_func *func, voi
 AVFilterContext *ff_filter_alloc(const AVFilter *filter, const char *inst_name)
 {
     AVFilterContext *ret;
+    int preinited = 0;
 
     if (!filter)
         return NULL;
@@ -708,6 +709,11 @@ AVFilterContext *ff_filter_alloc(const AVFilter *filter, const char *inst_name)
         if (!ret->priv)
             goto err;
     }
+    if (filter->preinit) {
+        if (filter->preinit(ret) < 0)
+            goto err;
+        preinited = 1;
+    }
 
     av_opt_set_defaults(ret);
     if (filter->priv_class) {
@@ -745,6 +751,8 @@ AVFilterContext *ff_filter_alloc(const AVFilter *filter, const char *inst_name)
     return ret;
 
 err:
+    if (preinited)
+        filter->uninit(ret);
     av_freep(&ret->inputs);
     av_freep(&ret->input_pads);
     ret->nb_inputs = 0;

libavfilter/avfilter.h

@@ -194,6 +194,21 @@ typedef struct AVFilter {
      *****************************************************************
      */
 
+    /**
+     * Filter pre-initialization function
+     *
+     * This callback will be called immediately after the filter context is
+     * allocated, to allow allocating and initing sub-objects.
+     *
+     * If this callback is not NULL, the uninit callback will be called on
+     * allocation failure.
+     *
+     * @return 0 on success,
+     *         AVERROR code on failure (but the code will be
+     *           dropped and treated as ENOMEM by the calling code)
+     */
+    int (*preinit)(AVFilterContext *ctx);
+
     /**
      * Filter initialization function.
      *