Merge the internal implementation of FilterGraph (#657)

Author: mthrok

src/libspdl/core/detail/ffmpeg/decoder.cpp

@@ -92,7 +92,7 @@ Generator<AVFramePtr> _decode_packet(
 Generator<AVFramePtr> decode_packets(
     AVCodecContextPtr& codec_ctx,
     const std::vector<AVPacket*>& packets,
-    std::optional<FilterGraph>& filter,
+    std::optional<FilterGraphImpl>& filter,
     bool flush) {
   auto packet_stream = _stream_packet(packets, flush);
   if (!filter) {
@@ -132,10 +132,7 @@ DecoderImpl<media>::DecoderImpl(
           codec.get_time_base(),
           cfg ? cfg->decoder : std::nullopt,
           cfg ? cfg->decoder_options : std::nullopt)),
-      filter_graph(get_filter<media>(
-          codec_ctx.get(),
-          filter_desc,
-          codec.get_frame_rate())) {}
+      filter_graph(filter_desc) {}
 
 template <MediaType media>
 Rational DecoderImpl<media>::get_output_time_base() const {

src/libspdl/core/detail/ffmpeg/decoder.h

@@ -24,7 +24,7 @@ namespace spdl::core::detail {
 template <MediaType media>
 class DecoderImpl {
   AVCodecContextPtr codec_ctx;
-  std::optional<FilterGraph> filter_graph;
+  std::optional<FilterGraphImpl> filter_graph;
 
  public:
   DecoderImpl(