pipewire-v4l2: More improvements

Keep separate map for fd to files so we can implement dup.

Filter out duplicates in enumfmt.

tryfmt should return the closest match.

Use pthread_once to init the global state.

Make things mostly work in GStreamer.

pipewire-v4l2/src/pipewire-v4l2.h

@@ -35,4 +35,4 @@ struct fops {
 	int (*munmap)(void *addr, size_t length);
 };
 
-extern const struct fops fops;
+const struct fops *get_fops(void);