lib/ffmpeg/Filter: add ParseSingleInOut()

Merge some duplicate code.

src/lib/ffmpeg/Filter.cxx

@@ -100,4 +100,17 @@ MakeAudioBufferSink(AVFilterGraph &graph_ctx)
 			    graph_ctx);
 }
 
+void
+FilterGraph::ParseSingleInOut(const char *filters, AVFilterContext &in,
+			      AVFilterContext &out)
+{
+	auto [inputs, outputs] = Parse(filters, {"out", in}, {"in", out});
+
+	if (inputs.get() != nullptr)
+		throw std::runtime_error("FFmpeg filter has an open input");
+
+	if (outputs.get() != nullptr)
+		throw std::runtime_error("FFmpeg filter has an open output");
+}
+
 } // namespace Ffmpeg

src/lib/ffmpeg/Filter.hxx

@@ -138,6 +138,9 @@ public:
 		return std::make_pair(std::move(inputs), std::move(outputs));
 	}
 
+	void ParseSingleInOut(const char *filters, AVFilterContext &in,
+			      AVFilterContext &out);
+
 	std::pair<FilterInOut, FilterInOut> Parse(const char *filters) {
 		AVFilterInOut *inputs, *outputs;
 		int err = avfilter_graph_parse2(graph, filters,