add Matrix4x3, needed for raytracing intrinsics

[Firestar99]

Instead of everyone having to implement their own Matrix4x3, we should just provide one with our #[spirv(matrix)] marker. Even our own tests have to implement their own:

rust-gpu/tests/compiletests/ui/spirv-attr/all-builtins.rs

Lines 8 to 15 in d1fce90

	#[derive(Clone, Copy)]
	#[spirv(matrix)]
	pub struct Matrix4x3 {
	pub x: glam::Vec3,
	pub y: glam::Vec3,
	pub z: glam::Vec3,
	pub w: glam::Vec3,
	}

Required for #392, as this removes OpTypeVector from affected intrinsics.