Promise / Future API with .then()

[almarklein]

Adds GPUPromise or GPUFuture, not sure yet whether to go for a more Pythonic or JS-like API.

The idea is that the async methods of the wgpu API will no longer have async in front, but instead return the promise object. This object can be awaited, sync waited, and also handled with a callback.

The callback-approach is new, and makes it possible to handle things asynchronously when not in async code. This opens up doors for improved performance, and a more sync workflow that's still portable. It will e.g. make the rendercanvas "bitmap" present-mode more performant, and allow us to resolve some issues related to timing events and draws.

Example of intended use:

>>> p = wgpu.gpu.request_adapter_async()
>>> p
<GPUPromise pending  at 0x107ead400>

>>> await p:
<wgpu.backends.wgpu_native.GPUAdapter object at 0x107eaeba0>

>>> p.wait()
<wgpu.backends.wgpu_native.GPUAdapter object at 0x107eaeba0>

>>> p.then(lambda adapter: print("got", adapter))
got <wgpu.backends.wgpu_native.GPUAdapter object at 0x107eaeba0>

[Korijn]

This seems to be pretty crucial to me!