[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>

.patch/opus/0001-disable-clang-runtime-neon.patch

@@ -1,28 +0,0 @@
-From cc15da16e533b2a801934eab2dfeaf3c3949a1dc Mon Sep 17 00:00:00 2001
-From: crueter <crueter@eden-emu.dev>
-Date: Mon, 8 Sep 2025 12:28:55 -0400
-Subject: [PATCH] [cmake] disable NEON runtime check on clang-cl
-
-When enabling runtime NEON checking for clang-cl, the linker would error out with `undefined symbol: __emit`, since clang doesn't actually implement this instruction. Therefore it makes sense to disable the runtime check by default on this platform, until either this is fixed or a clang-cl compatible intrinsic check is added (I don't have enough knowledge of MSVC to do this)
----
- cmake/OpusConfig.cmake | 7 ++++++-
- 1 file changed, 6 insertions(+), 1 deletion(-)
-
-diff --git a/cmake/OpusConfig.cmake b/cmake/OpusConfig.cmake
-index e9319fbad..d0f459e88 100644
---- a/cmake/OpusConfig.cmake
-+++ b/cmake/OpusConfig.cmake
-@@ -71,7 +71,12 @@ elseif(OPUS_CPU_ARM AND NOT OPUS_DISABLE_INTRINSICS)
-   opus_detect_neon(COMPILER_SUPPORT_NEON)
-   if(COMPILER_SUPPORT_NEON)
-     option(OPUS_USE_NEON "Option to enable NEON" ON)
--    option(OPUS_MAY_HAVE_NEON "Does runtime check for neon support" ON)
-+    if (MSVC AND CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-+      set(NEON_RUNTIME_CHECK_DEFAULT OFF)
-+    else()
-+      set(NEON_RUNTIME_CHECK_DEFAULT ON)
-+    endif()
-+    option(OPUS_MAY_HAVE_NEON "Does runtime check for neon support" ${NEON_RUNTIME_CHECK_DEFAULT})
-     option(OPUS_PRESUME_NEON "Assume target CPU has NEON support" OFF)
-     if(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
-       set(OPUS_PRESUME_NEON ON)