Avoid padding by compiler

For example, the compiler Clang 13 warned about padding struct 'struct ogg_opus_desc' with 4 bytes to align 'writing_pcm_pos' [-Wpadded].

formats/format_ogg_opus_open_source.c

@@ -61,7 +61,7 @@ struct ogg_opus_desc {
 	OggOpusComments *comments;
 #endif
 
-	int writing;
+	size_t writing;
 	off_t writing_pcm_pos;
 };