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/node-macro/src/codegen.rs

@@ -295,7 +295,8 @@ pub(crate) fn generate_node_code(parsed: &ParsedNodeFn) -> syn::Result<TokenStre
 
 	let cfg = crate::shader_nodes::modify_cfg(attributes);
 	let node_input_accessor = generate_node_input_references(parsed, fn_generics, &field_idents, &graphene_core, &identifier, &cfg);
-	let shader_entry_point = attributes.shader_node.as_ref().map(|n| n.codegen_shader_entry_point(parsed)).unwrap_or(Ok(TokenStream2::new()))?;
+	let ShaderTokens { shader_entry_point, gpu_node } = attributes.shader_node.as_ref().map(|n| n.codegen(parsed)).unwrap_or(Ok(ShaderTokens::default()))?;
+
 	Ok(quote! {
 		/// Underlying implementation for [#struct_name]
 		#[inline]
@@ -387,6 +388,8 @@ pub(crate) fn generate_node_code(parsed: &ParsedNodeFn) -> syn::Result<TokenStre
 		}
 
 		#shader_entry_point
+
+		#gpu_node
 	})
 }
 
@@ -589,7 +592,7 @@ fn generate_register_node_impl(parsed: &ParsedNodeFn, field_names: &[&Ident], st
 	})
 }
 
-use crate::shader_nodes::CodegenShaderEntryPoint;
+use crate::shader_nodes::{ShaderCodegen, ShaderTokens};
 use syn::visit_mut::VisitMut;
 use syn::{GenericArgument, Lifetime, Type};