Set filter graph threads to 1 for thread safety

Set the filter graph thread count to 1 in Graph.__cinit__ to prevent
potential thread safety issues when using filter graphs in multi-threaded
applications.

By default, FFmpeg's filter graph may use multiple threads for parallel
filter execution. However, this can cause race conditions or unexpected
behavior when the filter graph is accessed from Python code that isn't
designed for concurrent access.

Setting threads=1 ensures deterministic, single-threaded filter execution,
which is safer for typical PyAV usage patterns.

Author: root

av/filter/graph.pyx

@@ -14,6 +14,7 @@ from av.video.frame cimport VideoFrame
 cdef class Graph:
     def __cinit__(self):
         self.ptr = lib.avfilter_graph_alloc()
+        lib.av_opt_set_int(self.ptr, "threads", 1, 0)
         self.configured = False
         self._name_counts = {}