Use a default instead of blank channel map by default.

This commit fixes an issue where the optimized mono expansion path is
never hit.

miniaudio.h

 /*
 Audio playback and capture library. Choice of public domain or MIT-0. See license statements at the end of this file.
-miniaudio - v0.10.17 - 2020-08-28
+miniaudio - v0.10.18 - TBD
 
 David Reid - davidreidsoftware@gmail.com
 
@@ -1420,7 +1420,7 @@ extern "C" {
 
 #define MA_VERSION_MAJOR    0
 #define MA_VERSION_MINOR    10
-#define MA_VERSION_REVISION 17
+#define MA_VERSION_REVISION 18
 #define MA_VERSION_STRING   MA_XSTRINGIFY(MA_VERSION_MAJOR) "." MA_XSTRINGIFY(MA_VERSION_MINOR) "." MA_XSTRINGIFY(MA_VERSION_REVISION)
 
 #if defined(_MSC_VER) && !defined(__clang__)
@@ -2603,6 +2603,13 @@ Both input and output channel map buffers must have a capacity of at at least `c
 */
 MA_API void ma_channel_map_copy(ma_channel* pOut, const ma_channel* pIn, ma_uint32 channels);
 
+/*
+Copies a channel map if one is specified, otherwise copies the default channel map.
+
+The output buffer must have a capacity of at least `channels`. If not NULL, the input channel map must also have a capacity of at least `channels`.
+*/
+MA_API void ma_channel_map_copy_or_default(ma_channel* pOut, const ma_channel* pIn, ma_uint32 channels);
+
 
 /*
 Determines whether or not a channel map is valid.
@@ -37936,8 +37943,8 @@ MA_API ma_channel_converter_config ma_channel_converter_config_init(ma_format fo
     config.format      = format;
     config.channelsIn  = channelsIn;
     config.channelsOut = channelsOut;
-    ma_channel_map_copy(config.channelMapIn,  pChannelMapIn,  channelsIn);
-    ma_channel_map_copy(config.channelMapOut, pChannelMapOut, channelsOut);
+    ma_channel_map_copy_or_default(config.channelMapIn,  pChannelMapIn,  channelsIn);
+    ma_channel_map_copy_or_default(config.channelMapOut, pChannelMapOut, channelsOut);
     config.mixingMode  = mixingMode;
 
     return config;
@@ -40229,6 +40236,19 @@ MA_API void ma_channel_map_copy(ma_channel* pOut, const ma_channel* pIn, ma_uint
     }
 }
 
+MA_API void ma_channel_map_copy_or_default(ma_channel* pOut, const ma_channel* pIn, ma_uint32 channels)
+{
+    if (pOut == NULL || channels == 0) {
+        return;
+    }
+
+    if (pIn != NULL) {
+        ma_channel_map_copy(pOut, pIn, channels);
+    } else {
+        ma_get_standard_channel_map(ma_standard_channel_map_default, channels, pOut);
+    }
+}
+
 MA_API ma_bool32 ma_channel_map_valid(ma_uint32 channels, const ma_channel* pChannelMap)
 {
     if (pChannelMap == NULL) {
@@ -62485,6 +62505,11 @@ The following miscellaneous changes have also been made.
 /*
 REVISION HISTORY
 ================
+v0.10.18 - TBD
+  - Fix a bug in channel converter for s32 format.
+  - Change channel converter configs to use the default channel map instead of a blank channel map when no channel map is specified when initializing the
+    config. This fixes an issue where the optimized mono expansion path would never get used.
+
 v0.10.17 - 2020-08-28
   - Fix an error where the WAV codec is incorrectly excluded from the build depending on which compile time options are set.
   - Fix a bug in ma_audio_buffer_read_pcm_frames() where it isn't returning the correct number of frames processed.

research/ma_engine.c

@@ -28,7 +28,10 @@ void on_sound_loaded(ma_async_notification* pNotification)
 int main(int argc, char** argv)
 {
     ma_result result;
+    ma_resource_manager resourceManager;
+    ma_resource_manager_config resourceManagerConfig;
     ma_engine engine;
+    ma_engine_config engineConfig;
     ma_sound sound;
     ma_sound sound2;
     sound_loaded_notification loadNotification;
@@ -39,7 +42,18 @@ int main(int argc, char** argv)
         return -1;
     }
 
-    result = ma_engine_init(NULL, &engine);
+    resourceManagerConfig = ma_resource_manager_config_init();
+    //resourceManagerConfig.decodedFormat = ma_format_s16;
+    result = ma_resource_manager_init(&resourceManagerConfig, &resourceManager);
+    if (result != MA_SUCCESS) {
+        printf("Failed to initialize resource manager.\n");
+        return -1;
+    }
+
+    engineConfig = ma_engine_config_init_default();
+    engineConfig.pResourceManager = &resourceManager;
+
+    result = ma_engine_init(&engineConfig, &engine);
     if (result != MA_SUCCESS) {
         printf("Failed to initialize audio engine.\n");
         return -1;