Fix locking and threadsafety

Emit all notify in the context thread.
Handle error cases better.
Make sure we hold the lock when running the mainloop, this will cause
all of the signals to be emited with the lock. Make sure we call all
methods with the lock.

src/gst/gstpvsrc.h

@@ -65,6 +65,7 @@ struct _GstPulsevideoSrc {
   PvStream *stream;
   GstAllocator *fd_allocator;
 
+  GPollFunc poll_func;
   GMutex lock;
   GCond cond;
 };