Add a new experimental API to set additional image operands

[oisyn]

This PR adds a new experimental API to set additional image operands when sampling textures. Currently, it supports bias, lod, grad and sample_index operands.

Usage:

use spirv_std::spirv;
use spirv_std::{arch, image::sample_with, image::ImageWithMethods, Image, Sampler};

#[spirv(fragment)]
pub fn main(
    #[spirv(descriptor_set = 0, binding = 0)] sampler: &Sampler,
    #[spirv(descriptor_set = 0, binding = 1)] image1: &Image!(2D, type=f32, sampled, multisampled),
    #[spirv(descriptor_set = 0, binding = 2)] image2: &Image!(2D, type=f32, sampled),
    output: &mut glam::Vec4,
) {
    let t1 = image1.fetch_with(glam::IVec2::new(0, 0), sample_with::sample_index(1));
    let t2 = image2.sample_with(*sampler, glam::Vec2::new(0.5, 0.5), sample_with::bias(1.0));
    let t3 = image2.sample_with(*sampler, glam::Vec2::new(0.5, 0.5), sample_with::lod(2.0));
    let t4 = image2.sample_with(
        *sampler,
        glam::Vec2::new(0.5, 0.5),
        sample_with::grad(glam::Vec2::new(0.5, 0.5), glam::Vec2::new(0.5, 0.5)),
    );
    *output = t1 + t2 + t3 + t4;
}

Technically the API supports chaining of multiple operands, e.g. sample_with::bias(3.0).sample_index(1), although none of the currently supported operands can logically be combined.

The sampling methods are part of the image::ImageWithMethods trait, and it supports the following methods:

• fetch_with
• gather_with
• sample_with
• sample_depth_reference_with
• sample_with_project_coordinate_with
• sample_depth_reference_with_project_coordinate_with

Future improvements: support more operands, pervent combining mutually exclusive operands, and better error messaging when you use an unsupported operand on a specific sampling function.