Add Image::fetch_with_level

[caibear]

Fixes #360

I used the terminology level since that's what WGSL used, but it would also make sense to use lod since that's what SPIR-V and GLSL's texelFetch use.

[schell]

This looks good, thank you!

[Firestar99]

You can already fetch image texels with additional settings, like lod, bias, grad and sampleIndex. And naga for some reason requires explicitly fetching from lod 0, like below.

use spirv_std::image::sample_with::lod;
use spirv_std::image::{Image2d, ImageWithMethods};

mask_texture.fetch_with(frag_coord.xy().as_uvec2(), lod(0))

Also I'd call it lod since we're following spirv / glsl naming conventions.

You may also find this thing I've been working on useful: #280