Add MUSIC_AUDIO_NONE to MusicContextType and format fixes

- Useful to check the context type to see if the format is recognized. Defaulting to wav causes issues where formats are assumed to
be wav.

src/raudio.c

@@ -275,7 +275,8 @@ typedef struct tagBITMAPINFOHEADER {
 // NOTE: Depends on data structure provided by the library
 // in charge of reading the different file types
 typedef enum {
-    MUSIC_AUDIO_WAV = 0,
+    MUSIC_AUDIO_NONE = 0,
+    MUSIC_AUDIO_WAV,
     MUSIC_AUDIO_OGG,
     MUSIC_AUDIO_FLAC,
     MUSIC_AUDIO_MP3,