Added NonWritable + NonReadable Decorations. Fixes #689

[charles-r-earp]

Fixes #689.

This PR allows adding NonWritable and NonReadable decorations to StorageBuffers. This can be added to other types, Images for example. This is an opt in via attributes #[spirv(non_writable)] and #[spirv(non_readable)], and does not do any validation. Probably non_writable can be inferred from the function declaration but that can be addressed later.

It looks like this:

#![cfg_attr(
    target_arch = "spirv",
    no_std,
    feature(register_attr),
    register_attr(spirv)
)]
#![deny(warnings)]

use spirv_std::glam::UVec3;

#[repr(C)]
pub struct PushConsts {
    n: u32,
}

#[allow(unused)]
#[spirv(compute(threads(64)))]
pub fn add(
    #[spirv(global_invocation_id)] global_id: UVec3,
    #[spirv(storage_buffer, descriptor_set = 0, binding = 0, non_writable)] a: &[u32],
    #[spirv(storage_buffer, descriptor_set = 0, binding = 1, non_writable)] b: &[u32],
    #[spirv(storage_buffer, descriptor_set = 0, binding = 2, non_readable)] y: &mut [u32],
    #[spirv(push_constant)] push_consts: &PushConsts,
) {
    let gid = global_id.x as usize;
    if global_id.x < push_consts.n {
        y[gid] = a[gid] + b[gid];
    }
}

As far as validation, this could be done in the validation pass or through the type system, by adding a trait bound to the argument. But the former is probably the most robust way, but it has to do a recursive check against all the access chains as well. It would be nice if immutable arguments were declared non_writable automatically for known types at the very least. For example, &[T] where T: Copy should be able to be marked non_writable.

[khyperia]

Unfortunately, as stated in the readme, we're not able to accept major feature contributions right now :( - hopefully we can reopen this later when we're a little more stable!