Refactor Opus decoder

Created generic playback handler, first with Opus support. Other
decoders to follow. This is to remove duplicated code.

3DSX tested working with citra and N3DS.

Signed-off-by: Mahyar Koshkouei <deltabeard@users.noreply.github.com>

source/opus.h

@@ -1,5 +1,15 @@
 #include <opus/opusfile.h>
 
+void setOpus(struct decoder_fn* decoder);
+
+int initOpus(const char* file);
+
+uint32_t rateOpus(void);
+
+uint64_t decodeOpus(void* buffer);
+
+void exitOpus(void);
+
 int playOpus(const char* in);
 
 uint64_t fillOpusBuffer(OggOpusFile* opusFile, uint64_t samplesToRead,