Add specialization constants via #[spirv(spec_constant(id = 123))] x: u32 entry-point inputs.

[eddyb]

Quoting some of the documentation I added to docs/src/attributes.md:

Entry point inputs also allow access to SPIR-V "specialization constants",
which are each associated with an user-specified numeric "ID" (SPIR-V SpecId),
used to override them later ("specializing" the shader):

• in Vulkan: during pipeline creation, via VkSpecializationInfo
• in WebGPU: during pipeline creation, via GPUProgrammableStage#constants
  • note: WebGPU calls them "pipeline-overridable constants"
• in OpenCL: via clSetProgramSpecializationConstant() calls, before clBuildProgram()

If a "specialization constant" is not overriden, it falls back to its default
value, which is either user-specified (via default = ...), or 0 otherwise.

While only "specialization constants" of type u32 are currently supported, it's
always possible to manually create values of other types, from one or more u32s.

Example:

#[spirv(vertex)]
fn main(
    // Default is implicitly `0`, if not specified.
    #[spirv(spec_constant(id = 1))] no_default: u32,

    // IDs don't need to be sequential or obey any order.
    #[spirv(spec_constant(id = 9000, default = 123))] default_123: u32,

    // Assembling a larger value out of multiple `u32` is also possible.
    #[spirv(spec_constant(id = 100))] x_u64_lo: u32,
    #[spirv(spec_constant(id = 101))] x_u64_hi: u32,
) {
    let x_u64 = ((x_u64_hi as u64) << 32) | (x_u64_lo as u64);
}

---

There's not much else to it, quite bare-bones, but I wanted to have something for @VZout before the release.

[eddyb]

To test this for myself, I made it so that in example-runner-ash, you can now NumPad +/- to control sky-shader's "sun intensity", through a new specialization constant (and yes, it will rebuild the pipeline, and require recompiling the shaders in the driver, etc., every single time).

low "sun intensity"	high "sun intensity"

[fu5ha]

Cool :D