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,