Expose thread_type and nb_threads in filter graph

[lgeiger]

This PR exposes AVFilterGraph.nb_threads and AVFilterGraph.thread_type similar to what's currently done in the codec context.

[WyattBlue]

TheadType is too low level to be worth exposing. nb_threads should just be threads in Python land. nb is not a well known convention there.

[lgeiger]

Makes sense, I'll remove thread type, given that it only has one option at the moment anyway.

nb_threads should just be threads in Python land. nb is not a well known convention there.

Agreed, the nb prefix is not nice. I just named it like this to align with ffmpeg.
How do you feel about using thread_count similar to what's done in the codec context or do you think that's confusing and you prefer to stick with threads because there's no exact match of thread_count in AVFilterGraph?

PyAV/av/codec/context.py

Lines 629 to 642 in 37a6ece

	@property
	def thread_count(self):
	"""How many threads to use; 0 means auto.
	Wraps :ffmpeg:`AVCodecContext.thread_count`.
	"""
	return self.ptr.thread_count
	@thread_count.setter
	def thread_count(self, value: cython.int):
	if self.is_open:
	raise RuntimeError("Cannot change thread_count after codec is open.")
	self.ptr.thread_count = value

[WyattBlue]

I prefer threads because thread_count is not a C field.