Non uniform for everything!

[Firestar99]

Adds the NonUniform declaration to pretty much everything I could think of, 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.

To have NonUniform declared everywhere, you just need to enable it's associated capability: SpirvBuilder::capability(Capability::ShaderNonUniform)

closes #120

[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)

[LegNeato]

I'm personally fine with either a linking stage or copying 200 lines (or a macro so no need to copy?)

[Firestar99]

I've taken the linking stage filter route and added the NonUniform decoration it everywhere, hopefully didn't miss anything. Tested it on my project too.

I've added some tests in branch byte_adressable_buffer_from_runtime_array, which work on spv1.3 but fail on vulkan1.2. That behavior doesn't change on master, so unrelated to this PR. Weirdly enough, that's exactly how I'm using them in my project, yet it compiles with vulkan1.2 just fine.

[Firestar99]

@LegNeato this is ready to review, in case it has gone under. Just needed a rebase due to compiletest conflicts.

[LegNeato]

LGTM!