ui/prores_aw_handler: Workaround for FFmpeg bug with ProRes in Matroska

In current FFmpeg, whenever Matroska with ProRes is demuxed it creates an atom that is just 8 bytes short of the true size necessary.

We can work around this by padding the actual packet by 8 0x00 bytes, which should result in older FFmpeg versions working fine.

An FFmpeg patch is available: http://ffmpeg.org/pipermail/ffmpeg-devel/2019-September/250724.html

source/ui/prores_aw_handler.cpp

@@ -112,3 +112,17 @@ void obsffmpeg::ui::prores_aw_handler::log_options(obs_data_t* settings, const A
 			PLOG_INFO("[%s]   Profile: %s", codec->name, ptr->name);
 	}
 }
+
+void obsffmpeg::ui::prores_aw_handler::process_avpacket(AVPacket& packet, const AVCodec* codec, AVCodecContext* context)
+{
+	//FFmpeg Bug:
+	// When ProRes content is stored in Matroska, FFmpeg strips the size
+	// from the atom. Later when the ProRes content is demuxed from Matroska,
+	// FFmpeg creates an atom with the incorrect size, as the ATOM size
+	// should be content + atom, but FFmpeg set it to only be content. This
+	// difference leads to decoders to be off by 8 bytes.
+	//Fix (until FFmpeg stops being broken):
+	// Pad the packet with 8 bytes of 0x00.
+
+	av_grow_packet(&packet, 8);
+}

source/ui/prores_aw_handler.hpp

@@ -48,6 +48,8 @@ namespace obsffmpeg {
 
 			virtual void log_options(obs_data_t* settings, const AVCodec* codec,
 			                         AVCodecContext* context) override;
+
+			virtual void process_avpacket(AVPacket& packet, const AVCodec* codec, AVCodecContext* context) override;
 		};
 	} // namespace ui
 } // namespace obsffmpeg