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

[almarklein]

I implemented this in #618. Some context I found along the way: It's not a matter of just calling _draw_frame_and_present(). Well, it is for glfw, for qt that'd result in flicker, because the drawing is done outside qt's draw event. Fortunately, the systems that need it (qt and wx) both have a method to force a repaint. For the jupyter backend we can also force the drawing, which will basically feed images to the browser until the websocket buffer fills up.

Another, more friendly, approach for supporting uses-cases for drawing inside a loop, could be to have an async method, that can request a draw and then await for that draw to have occurred.