Descriptor Heap Support

[codecnotsupported]

Whilst descriptor heap can be used with regular descriptor sets/bindings.
It also allows for untyped access which is much more flexible.

// VkPhysicalDeviceDescriptorHeapPropertiesEXT::bufferDescriptorSize == 16 (0x10)
layout(descriptor_heap) buffer SSBO {
    vec4 payload;
} s_buffers[];

void main() {
    s_buffers[6].payload = vec4(1.0);
}

https://github.khronos.org/SPIRV-Registry/extensions/EXT/SPV_EXT_descriptor_heap.html
https://github.khronos.org/SPIRV-Registry/extensions/KHR/SPV_KHR_untyped_pointers.html
https://github.com/KhronosGroup/SPIRV-Guide/blob/main/chapters/untyped_pointers.md

https://docs.vulkan.org/guide/latest/descriptor_heap.html#_untyped_shader_model
https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_descriptor_heap.html
https://docs.vulkan.org/features/latest/features/proposals/VK_EXT_descriptor_heap.html

[Firestar99]

You're the third to ask me about descriptor heaps, ping @FacelessTiger :D

On branch asm_fn_ptr I made prototypes to declare the entry point in asm! blocks, which would allow end users with advanced entry point requirements to directly emit whatever SPIR-V they want to output. Example:

rust-gpu/tests/compiletests/ui/spirv-attr/entry/entry1_global_asm.rs

Lines 20 to 57 in 3c72edc

	#[inline]
	pub fn main(input: Vec4, out: &mut Vec4) {
	*out = input + 1.;
	}
	pub fn entry() {
	unsafe {
	let mut input = Vec4::default();
	asm! {
	"%input = OpVariable typeof{input} Input",
	"OpName %input \"input\"",
	"OpDecorate %input Location 0",
	"%tmp = OpLoad typeof*{input} %input",
	"OpStore {input} %tmp",
	input = in(reg) &mut input,
	};
	let mut output = Vec4::default();
	main(input, &mut output);
	asm! {
	"%output = OpVariable typeof{output} Output",
	"OpName %output \"output\"",
	"OpDecorate %output Location 0",
	"%tmp = OpLoad _ {output}",
	"OpStore %output %tmp",
	output = in(reg) &output,
	};
	}
	}
	/// we don't support `global_asm!` yet
	pub fn non_global_asm() {
	unsafe {
	asm! {
	"OpEntryPoint Vertex {entry} \"main\"",
	entry = in(reg) entry,
	}
	}
	}

Primarily, I want to move our entry point system from being a compiler internal thing to a "simple" proc macro you can expand, inspect and modify more easily. We could also have a proc macro for descriptor heaps, or you could use said assembly directly.

I've had some trouble getting buffers to work, but got a hint that this works so may revisit this soon.

[Firestar99]

The compiletest prototypes in #534 may be interesting reference on how this could be implemented