Based on the work in #6717, plus additional adaptions mentioned in
https://github.com/gfx-rs/wgpu/blob/master/CHANGELOG.md#wgpu-013-2022-06-30,
and more not-mentioned but required changes.
Also bump `wasm-bindgen` to `0.2.81` (along with its helper crates), as
required by the new `wgpu` version.
Note that I don't fully understand some of the required changes, notably:
* `wgpu::PresentMode::Mailbox` no longer works on my machine (Windows 11) -
The `wgpu` documentation says that `wgpu::PresentMode::Fifo` is the
only guaranteed to be supported, so I switched over to it instead.
* `self.staging_belt.recall()` doesn't return a `Future` anymore -
I assume it became synchronous so I simply removed the `executor`
from there.
relrelb