[audio] Replace libopus with FFmpeg (#4169)

FFmpeg offers two opus decoders:
- libopus, this one is fed the same data (that affects output) as the previous libopus decoder had as well
- native opus decoder: this only takes Float32 samples, we then need to invoke swrescale to translate between s16 (what switch sends) and float32 (what FFmpeg expects), the process is computationally more expensive, but trades off neatly as the native opus decoder is very optimised

It also relegates the handling to FFmpeg, which simplifies existing code (more concretely, by using the "fill/drain" scheme for packets/frames)

Closes #2897

Signed-off-by: lizzie <lizzie@eden-emu.dev>
Co-authored-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4169
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>
Reviewed-by: Maufeat <sahyno1996@gmail.com>

src/core/CMakeLists.txt

@@ -1199,7 +1199,6 @@ else()
     endif()
 endif()
 
-target_include_directories(core PRIVATE ${OPUS_INCLUDE_DIRS})
 target_link_libraries(core PUBLIC common PRIVATE audio_core hid_core network video_core nx_tzdb tz)
 
 if (BOOST_NO_HEADERS)