Enable support for 3D textures

[ryanw]

Currently when attempting to create a 3D texture it panics with: internal error: entered unreachable code

But simply adding D3 to the match seems to work just fine. I've tested as far as creating a 3D texture and inspecting it using RenderDoc.

Here's an example of creating a texture that is fixed by this change:

device.create_texture(&wgpu::TextureDescriptor {
        label: Some("Voxel texture"),
        size: wgpu::Extent3d {
                width: 128,
                height: 128,
                depth_or_array_layers: 128,
        },
        mip_level_count: 1,
        sample_count: 1,
        dimension: wgpu::TextureDimension::D3,
        format: wgpu::TextureFormat::Rgba8UnormSrgb,
        usage: wgpu::TextureUsages::TEXTURE_BINDING
                | wgpu::TextureUsages::COPY_SRC
                | wgpu::TextureUsages::COPY_DST
                | wgpu::TextureUsages::RENDER_ATTACHMENT,
});

A 3D texture inside RenderDoc:

[kvark]

Unfortunately, it's not that simple. The matter of fact is that 3D textures aren't really renderable in baseline WebGPU. Creating 2D views of them is not portable. See gpuweb/gpuweb#2303 for a starting point.

In this case, we are missing a validation check that would see that the RENDER_ATTACHMENT usage is requested for a 3D texture and complain. This code path here is truly unreachable therefore.

[kvark]

I filed gpuweb/gpuweb#2603 to address this in the spec. We'll still need to add the relevant validation code on our side.

FYI, your workaround should be making it a 2D array texture instead of 3D.