audio: Added basic WAVE_FORMAT_EXTENSIBLE support to .wav loader.

This is just enough to get you through a file that just used the extended
header for float or int data. It doesn't handle all the other things that
you expect from this header, like 24-bit samples inside a 32-bit container
or speaker masks.

src/audio/SDL_wave.h

@@ -38,6 +38,7 @@
 #define IEEE_FLOAT_CODE 0x0003
 #define IMA_ADPCM_CODE  0x0011
 #define MP3_CODE        0x0055
+#define EXTENSIBLE_CODE 0xFFFE
 #define WAVE_MONO       1
 #define WAVE_STEREO     2
 
@@ -64,4 +65,13 @@ typedef struct Chunk
     Uint8 *data;
 } Chunk;
 
+typedef struct WaveExtensibleFMT
+{
+    WaveFMT format;
+    Uint16 size;
+    Uint16 validbits;
+    Uint32 channelmask;
+    Uint8 subformat[16];  /* a GUID. */
+} WaveExtensibleFMT;
+
 /* vi: set ts=4 sw=4 expandtab: */