Add max_b_frames property to codec context

Allows an application to control the maximum number of B frames,
irrespective of what other settings or preset is used.

av/video/codeccontext.pyx

@@ -162,3 +162,10 @@ cdef class VideoCodecContext(CodecContext):
     property coded_height:
         def __get__(self):
             return self.ptr.coded_height
+
+    property max_b_frames:
+        def __get__(self):
+            return self.ptr.max_b_frames
+
+        def __set__(self, value):
+            self.ptr.max_b_frames = value