Shaders: runtime and shader node codegen (#2985)

* shader-rt: initial

* shader-rt: fix recursion when generating shader node

* shader-rt: replace gpu node's args and ret types with `Raster<GPU>`

* shader-rt: properly cfg out the gpu node

* shader-rt: fix `impl Context` in the wrong places

* shader-rt: disable gpu blend node, needs two images

* shader-rt: connect shader runtime

* shader-rt: pass WgpuExecutor by reference

* shader-rt: correct bindings with derpy arg buffer

* shader-rt: manual pipeline layout, fixing errors when bindings got DCE'd

* shader-rt: correct RT format, working invert gpu node

* shader-rt: cleanup codegen with common sym struct

* shader-rt: correct arg buffer handling

* shader-nodes feature: put shader nodes behind feature gate

* shader-nodes feature: rename any `gpu_node` to `shader-node`

* shaders-rt: fix wgpu label name

* shaders-rt: explain fullscreen_vertex coordinates with a drawing

node-graph/graster-nodes/src/fullscreen_vertex.rs

@@ -0,0 +1,29 @@
+use glam::{Vec2, Vec4};
+use spirv_std::spirv;
+
+/// webgpu NDC is like OpenGL: (-1.0 .. 1.0, -1.0 .. 1.0, 0.0 .. 1.0)
+/// https://www.w3.org/TR/webgpu/#coordinate-systems
+///
+/// So to make a fullscreen triangle around a box at (-1..1):
+///
+/// ```norun
+/// 3  +
+///    |\
+/// 2  |  \
+///    |    \
+/// 1  +-----+
+///    |     |\
+/// 0  |  0  |  \
+///    |     |    \
+/// -1 +-----+-----+
+///   -1  0  1  2  3
+/// ```
+const FULLSCREEN_VERTICES: [Vec2; 3] = [Vec2::new(-1., -1.), Vec2::new(-1., 3.), Vec2::new(3., -1.)];
+
+#[spirv(vertex)]
+pub fn fullscreen_vertex(#[spirv(vertex_index)] vertex_index: u32, #[spirv(position)] gl_position: &mut Vec4) {
+	// broken on edition 2024 branch
+	// let vertex = unsafe { *FULLSCREEN_VERTICES.index_unchecked(vertex_index as usize) };
+	let vertex = FULLSCREEN_VERTICES[vertex_index as usize];
+	*gl_position = Vec4::from((vertex, 0., 1.));
+}