[wgpu-core] split command encoders from command buffers

[teoxoy]

Fixes #7812 and partially Bug 1972949.

[andyleiserson]

Nice, thank you for taking care of this.

I left some minor comments but nothing that I see as blocking. I pushed a branch with all of my CommandEncoderStatus::Consumed changes here.

[andyleiserson]

nit: since the type names are kind of unwieldy and are repeated a few times, maybe it's worth importing the components or defining type aliases for the two identity manager types?

[andyleiserson]

Does it make sense to add a new CommandEncoderStatus::Consumed for the status of an encoder that has been finished? (vs. Finished, which after this change is the CommandEncoderStatus of a valid command buffer). Distinguishing it as a separate state makes it easier to distinguish "there's a bug in wgpu, I didn't expect to find this state here" from an application doing something illegal that results in objects being invalid.

[andyleiserson]

This is actually a comment about CommandEncoderStatus::finish, below. We could change the API to reflect that the operation is no longer a mutation of the common state of encoders/buffers, but is more like an update to the state of a command encoder that also returns some information that will be given to the new command buffer. (Eventually, we could even split the buffer states out of CommandEncoderStatus into CommandBufferStatus, but that's a bigger refactoring that is probably better not to combine with this PR.)

Maybe something like:

    fn finish(&mut self) -> Self {
        // Replace our state with `Consumed`, and return either the inner
        // state or an error, to be transferred to the command buffer.
        match mem::replace(self, Self::Consumed) {
            Self::Recording(mut inner) => {
                if let Err(err) = inner.encoder.close_if_open() {
                    Self::Error(err.into())
                } else {
                    // Note: if we want to stop tracking the swapchain texture view,
                    // this is the place to do it.
                    Self::Finished(inner)
                }
            }
            Self::Consumed | Self::Finished(_) => Self::Error(EncoderStateError::Ended.into()),
            Self::Locked(_) => Self::Error(EncoderStateError::Locked.into()),
            st @ Self::Error(_) => st,
            Self::Transitioning => unreachable!(),
        }
    }