entry: apply NonWritable to read-only StorageBuffers.

[eddyb]

This PR applies NonWritable to #[spirv(storage_buffer)] x: &T (if T doesn't have interior mutability).

• fixes Support for NonReadable, NonWritable #689
  • (assuming we want to postpone NonReadable)
    ^{(also we should maybe do something different for storage images?)}
  • limited to StorageBuffer as, according to spirv-val, only it and Uniform allow NonWritable, and Uniform buffers are always read-only anyway
• motivated by: Validation panic when loading a shader that accesses lighting data Bevy-Rust-GPU/bevy-rust-gpu#13
  • (more specifically, wgpu read-only-storage buffers are stricter than their Vulkan counterparts)