Blocking render call

[Korijn]

Make it possible to render a new frame synchronously; in other words, a public API to call canvas._draw_frame_and_present directly. Do we have this?

Relates to pygfx/pygfx#667

[almarklein]

We don't have this for normal canvases, but the offscreen canvas does exactly that:

wgpu-py/wgpu/gui/offscreen.py

Lines 202 to 209 in b54f628

	def draw(self):
	"""Perform a draw and get the resulting image.
	The image array is returned as an NxMx4 memoryview object.
	This object can be converted to a numpy array (without copying data)
	using ``np.asarray(arr)``.
	"""
	return self._draw_frame_and_present()

Users could use the private canvas._draw_frame_and_present for now. I'm a little worried, if we make this a public method, how this affects e.g. async code, once we implement that.

[almarklein]

Relates to #391