ffmpeg: Add ListFormats and ListEncoders

These two functions allow the frontend to get a list of encoders/formats and their specific options.

Retrieving the options is harder than it sounds due to FFmpeg's strange AVClass and AVOption system. For example, for integer and flags options, 'named constants' can be set. They are of type `AV_OPT_TYPE_CONST` and are categoried according to the `unit` field. An option can recognize all constants of the same `unit`.

src/core/dumping/ffmpeg_backend.h

@@ -9,6 +9,7 @@
 #include <limits>
 #include <memory>
 #include <mutex>
+#include <set>
 #include <thread>
 #include <vector>
 #include "common/common_types.h"
@@ -19,6 +20,7 @@
 extern "C" {
 #include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>
+#include <libavutil/opt.h>
 #include <libswresample/swresample.h>
 #include <libswscale/swscale.h>
 }
@@ -188,4 +190,43 @@ private:
     Common::Event processing_ended;
 };
 
+/// Struct describing encoder/muxer options
+struct OptionInfo {
+    std::string name;
+    std::string description;
+    AVOptionType type;
+    std::string default_value;
+    struct NamedConstant {
+        std::string name;
+        std::string description;
+        s64 value;
+    };
+    std::vector<NamedConstant> named_constants;
+
+    // If this is a scalar type
+    double min;
+    double max;
+};
+
+/// Struct describing an encoder
+struct EncoderInfo {
+    std::string name;
+    std::string long_name;
+    AVCodecID codec;
+    std::vector<OptionInfo> options;
+};
+
+/// Struct describing a format
+struct FormatInfo {
+    std::string name;
+    std::string long_name;
+    std::vector<std::string> extensions;
+    std::set<AVCodecID> supported_video_codecs;
+    std::set<AVCodecID> supported_audio_codecs;
+    std::vector<OptionInfo> options;
+};
+
+std::vector<EncoderInfo> ListEncoders(AVMediaType type);
+std::vector<FormatInfo> ListFormats();
+
 } // namespace VideoDumper