Queue.submit API changes

[nical]

Followup from #5028

Quoting @cwfitzgerald:

[..] To be consistent we need to make the submission index hold an Option on the direct backend, make poll a no-op if the submission index is None, then send all submit errors down the error sync.

This sucks, but the futures api is a better one, so will be resolved when we can figure that out.

Once this is done we can make it so queue.submit doesn't panic on errors and add the following test:

// Test that destroying a used resource between command buffer recording and submission fails gracefully.
#[gpu_test]
static TEXTURE_DESTROY_BEFORE_SUBMIT: GpuTestConfiguration = GpuTestConfiguration::new().run_sync(|ctx| {
    let descriptor = wgpu::TextureDescriptor {
        label: None,
        size: wgpu::Extent3d {
            width: 128,
            height: 128,
            depth_or_array_layers: 1,
        },
        mip_level_count: 1,
        sample_count: 1, // multisampling is not supported for clear
        dimension: wgpu::TextureDimension::D2,
        format: wgpu::TextureFormat::Rgba8Snorm,
        usage: wgpu::TextureUsages::COPY_DST | wgpu::TextureUsages::COPY_SRC | wgpu::TextureUsages::TEXTURE_BINDING,
        view_formats: &[],
    };

    let texture_1 = ctx.device.create_texture(&descriptor);
    let texture_2 = ctx.device.create_texture(&descriptor);

    let mut encoder = ctx.device.create_command_encoder(&wgpu::CommandEncoderDescriptor::default());
    encoder.copy_texture_to_texture(
        wgpu::ImageCopyTexture {
            texture: &texture_1,
            mip_level: 0,
            origin: wgpu::Origin3d::ZERO,
            aspect: wgpu::TextureAspect::All,
        },
        wgpu::ImageCopyTexture {
            texture: &texture_2,
            mip_level: 0,
            origin: wgpu::Origin3d::ZERO,
            aspect: wgpu::TextureAspect::All,            
        },
        wgpu::Extent3d {
            width: 128,
            height: 128,
            depth_or_array_layers: 1,
        }
    );

    texture_1.destroy();
    texture_2.destroy();

    fail(&ctx.device, || {
        ctx.queue.submit([encoder.finish()]);
    });
});

We'd like a similar test for buffers as well.