Non uniform for everything!

[Firestar99]

Adds the NonUniform declaration to pretty much everything, even if it's not required. I don't think this will have a huge performance impact, as Nvidia and Intel don't seem to care about this flag, their accesses are always non-uniform, and Amd's driver is really good at figuring out whether something is uniform / scalar across the workgroup / subgroup or not.

You will have to declare SpirvBuilder::capability(Capability::ShaderNonUniform) for it to work though.

Feel free to cherry-pick these commits if you do need non-uniform now and can't wait until rust-gpu has figured out a proper solution.

[LegNeato]

I wonder if we should land this behind an off-by-default flag? Or maybe that is more trouble than it is worth.

[Firestar99]

I'm shocked I haven't thought of this earlier... then I wouldn't have to keep updating this PR constantly

[Firestar99]

@LegNeato Buffers are easy, but for images, we can't just cfg within an asm!:

asm! {
    // error[E0658]: attributes on expressions are experimental
    // error: removing an expression is not supported in this position
    #[cfg(feature = "non-uniform")]
    "OpDecorate %image NonUniform"
    ...
}

We can't put it in a separate asm! either as we need to decorate the access instruction and all it's parameters (as a precaution). The correct way would be to cfg the entire asm! and duplicate all the image access asm!, about 200 loc.

Would you mind if I just add a linking stage instead to remove all OpDecoration if the target feature ShaderNonUniform is not present? (cc @eddyb)