The most important one being adding a mutex to protect the format_context. Apparently it wasn't thread safe (as one'd expect) but I didn't think about that.
Should fix some of the strange issues happening with MP4 muxers, etc.
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`.
Previously, we just used the native sample rate for encoding. However, some encoders like libmp3lame doesn't support it. Therefore, we now use a supported sample rate (preferring the native one if possible).
FFmpeg requires audio data to be sent in a sequence of frames, each containing the same specific number of samples. Previously, we buffered input samples in FFmpegBackend. However, as the source and destination sample rates can now be different, we should buffer resampled data instead. swresample have an internal input buffer, so we now just forward all data to it and 'gradually' receive resampled data, at most one frame_size at a time. When there is not enough resampled data to form a frame, we will record the current offset and request for less data on the next call.
Additionally, this commit also fixes a flaw. When an encoder supports variable frame sizes, its frame size is reported to be 0, which breaks our buffering system. Now we treat variable frame size encoders as having a frame size of 160 (the size of a HLE audio frame).
