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opl: Use SDL_mixer post-mix hook for OPL output.

Using Mix_HookMusic() to generate the OPL output stream works fine if
you only ever play OPL music and don't want to use the normal music
output functionality of SDL_mixer. However, now that it's possible to
use substitute music packs with any music output type, this is no longer
the case. Using Mix_HookMusic() disables normal output from eg. Ogg and
FLAC playback. As an alternative, use the post-mix hook and mix the
output from the OPL emulator there.

Cleanup fix as part of #440.

--- a/opl/opl_sdl.c

+++ b/opl/opl_sdl.c

@@ -75,7 +75,7 @@

 // Temporary mixing buffer used by the mixing callback.

-static int32_t *mix_buffer = NULL;

+static uint8_t *mix_buffer = NULL;

 // Register number that was written.

@@ -155,36 +155,32 @@

 // Call the OPL emulator code to fill the specified buffer.

-static void FillBuffer(int16_t *buffer, unsigned int nsamples)

+static void FillBuffer(uint8_t *buffer, unsigned int nsamples)

 {

     // This seems like a reasonable assumption.  mix_buffer is

     // 1 second long, which should always be much longer than the

     // SDL mix buffer.

-

     assert(nsamples < mixing_freq);

-    OPL3_GenerateStream(&opl_chip, buffer, nsamples);

+    // OPL output is generated into temporary buffer and then mixed

+    // (to avoid overflows etc.)

+    OPL3_GenerateStream(&opl_chip, (Bit16s *) mix_buffer, nsamples);

+    SDL_MixAudioFormat(buffer, mix_buffer, AUDIO_S16SYS, nsamples * 4,

+                       SDL_MIX_MAXVOLUME);

 }

 // Callback function to fill a new sound buffer:

-static void OPL_Mix_Callback(void *udata,

-                             Uint8 *byte_buffer,

-                             int buffer_bytes)

+static void OPL_Mix_Callback(void *udata, Uint8 *buffer, int len)

 {

-    int16_t *buffer;

-    unsigned int buffer_len;

-    unsigned int filled = 0;

+    unsigned int filled, buffer_samples;

-    // Buffer length in samples (quadrupled, because of 16-bit and stereo)

-

-    buffer = (int16_t *) byte_buffer;

-    buffer_len = buffer_bytes / 4;

-

     // Repeatedly call the OPL emulator update function until the buffer is

     // full.

+    filled = 0;

+    buffer_samples = len / 4;

-    while (filled < buffer_len)

+    while (filled < buffer_samples)

     {

         uint64_t next_callback_time;

         uint64_t nsamples;

@@ -197,7 +193,7 @@

         if (opl_sdl_paused || OPL_Queue_IsEmpty(callback_queue))

         {

-            nsamples = buffer_len - filled;

+            nsamples = buffer_samples - filled;

         }

         else

         {

@@ -206,9 +202,9 @@

             nsamples = (next_callback_time - current_time) * mixing_freq;

             nsamples = (nsamples + OPL_SECOND - 1) / OPL_SECOND;

-            if (nsamples > buffer_len - filled)

+            if (nsamples > buffer_samples - filled)

             {

-                nsamples = buffer_len - filled;

+                nsamples = buffer_samples - filled;

             }

         }

@@ -216,7 +212,7 @@

         // Add emulator output to buffer.

-        FillBuffer(buffer + filled * 2, nsamples);

+        FillBuffer(buffer + filled * 4, nsamples);

         filled += nsamples;

         // Invoke callbacks for this point in time.

@@ -340,10 +336,9 @@

         return 0;

     }

-    // Mix buffer:

+    // Mix buffer: four bytes per sample (16 bits * 2 channels):

+    mix_buffer = malloc(mixing_freq * 4);

-    mix_buffer = malloc(mixing_freq * sizeof(uint32_t) * 2);

-

     // Create the emulator structure:

     OPL3_Reset(&opl_chip, mixing_freq);

@@ -352,8 +347,10 @@

     callback_mutex = SDL_CreateMutex();

     callback_queue_mutex = SDL_CreateMutex();

-    // TODO: This should be music callback? or-?

-    Mix_HookMusic(OPL_Mix_Callback, NULL);

+    // Set postmix that adds the OPL music. This is deliberately done

+    // as a postmix and not using Mix_HookMusic() as the latter disables

+    // normal SDL_mixer music mixing.

+    Mix_SetPostMix(OPL_Mix_Callback, NULL);

     return 1;

 }