lavfi: rewrite nullsrc using the code in vsrc_testsrc.c

Factorize code, extend the functionality of the filter, and make it
return empty buffers. This is useful for filters which ignore the input
frames content.

This is also changing the syntax of the nullsrc source, and dropping the
framerate expression evaluation, which does not look particularly useful.

libavfilter/vsrc_testsrc.c

@@ -148,6 +148,46 @@ static int request_frame(AVFilterLink *outlink)
     return 0;
 }
 
+#if CONFIG_NULLSRC_FILTER
+
+static const char *nullsrc_get_name(void *ctx)
+{
+    return "nullsrc";
+}
+
+static const AVClass nullsrc_class = {
+    .class_name = "NullSourceContext",
+    .item_name  = nullsrc_get_name,
+    .option     = testsrc_options,
+};
+
+static void nullsrc_fill_picture(AVFilterContext *ctx, AVFilterBufferRef *picref) { }
+
+static av_cold int nullsrc_init(AVFilterContext *ctx, const char *args, void *opaque)
+{
+    TestSourceContext *test = ctx->priv;
+
+    test->class = &nullsrc_class;
+    test->fill_picture_fn = nullsrc_fill_picture;
+    return init(ctx, args, opaque);
+}
+
+AVFilter avfilter_vsrc_nullsrc = {
+    .name        = "nullsrc",
+    .description = NULL_IF_CONFIG_SMALL("Null video source, return unprocessed video frames."),
+    .init       = nullsrc_init,
+    .priv_size  = sizeof(TestSourceContext),
+
+    .inputs    = (AVFilterPad[]) {{ .name = NULL}},
+    .outputs   = (AVFilterPad[]) {{ .name = "default",
+                                    .type = AVMEDIA_TYPE_VIDEO,
+                                    .request_frame = request_frame,
+                                    .config_props  = config_props, },
+                                  { .name = NULL}},
+};
+
+#endif /* CONFIG_NULLSRC_FILTER */
+
 #if CONFIG_TESTSRC_FILTER
 
 static const char *testsrc_get_name(void *ctx)