API CHANGE: Add an onCopy callback to ma_data_source_vtable.

This is used for making a copy of a data source.

Data sources are not required to support copying, in which case this
callback can be set to NULL, or it can return MA_NOT_IMPLEMENTED.

This commit just sets up the infrastructure. As of this commit, no data
sources actually implement this, however future commits will be
introducing support in stages.

extras/decoders/libopus/miniaudio_libopus.c

@@ -49,6 +49,7 @@ static ma_data_source_vtable ma_gDataSourceVTable_libopus =
 {
     ma_libopus_ds_sizeof,
     ma_libopus_ds_uninit,
+    NULL,   /* onCopy. Copying is not supported. */
     ma_libopus_ds_read,
     ma_libopus_ds_seek,
     ma_libopus_ds_get_data_format,

extras/decoders/libopus/miniaudio_libopus.h

@@ -1,8 +1,10 @@
 /*
-This implements a data source that decodes Opus streams via libopus + libopusfile
+This implements a data source that decodes Opus streams via libopus + libopusfile.
 
 This object can be plugged into any `ma_data_source_*()` API and can also be used as a custom
 decoding backend. See the custom_decoder example.
+
+This does not support copying.
 */
 #ifndef miniaudio_libopus_h
 #define miniaudio_libopus_h

extras/decoders/libvorbis/miniaudio_libvorbis.c

@@ -52,6 +52,7 @@ static ma_data_source_vtable ma_gDataSourceVTable_libvorbis =
 {
     ma_libvorbis_ds_sizeof,
     ma_libvorbis_ds_uninit,
+    NULL,   /* onCopy. Copying is not supported. */
     ma_libvorbis_ds_read,
     ma_libvorbis_ds_seek,
     ma_libvorbis_ds_get_data_format,

extras/decoders/libvorbis/miniaudio_libvorbis.h

@@ -1,8 +1,10 @@
 /*
-This implements a data source that decodes Vorbis streams via libvorbis + libvorbisfile
+This implements a data source that decodes Vorbis streams via libvorbis + libvorbisfile.
 
 This object can be plugged into any `ma_data_source_*()` API and can also be used as a custom
 decoding backend. See the custom_decoder example.
+
+This does not support copying.
 */
 #ifndef miniaudio_libvorbis_h
 #define miniaudio_libvorbis_h