Use a shader graph as a node in other shaders (#303)

A calculation needed in several shaders (a greyscale, a noise, an
outline) had to be built again in every one of them. A shader
function keeps it as an asset of its own, called like a behavior is
since #16.

- New asset type `shaderfunction` ("Shader function"), edited in the
  Shader tab. The panel beside the graph shows no picture, the
  readout counts its inputs and outputs.
- Two new nodes: `Function input` and `Function output`. Their
  declared ports, numbers or colours only, are the ports of every
  call. One of each per function, like Start and End.
- The call is node `ShaderCall`, named after the function. The
  palette lists every function, it can be dragged in from the Assets
  panel, and a double-click opens it.
- The compiler sets the function in where it is called
  (`inlineFunctions`). It renames the copied nodes `<call>/<node>`
  and turns an unconnected port into a constant. Passes, Time, the
  Edge cut and the corner walk then see a plain graph.
- A problem inside a function is reported at the call, in the
  function's words. The function's own tab checks it on its own
  (`checkShaderFunction`).
- `ShaderLibrary` keys a shader on its graph and on every function it
  calls, however deep, so a change reaches every caller.
- The port commands mirror a new, renamed or removed port into every
  call, as for a behavior's Start and End. `NodeDef.portKinds` holds
  the two ends to Number and Color.
- As agreed in the issue: no `Property`, `Color property`, Output or
  Vertex output in a function, and `Time` is the caller's clock.
- Own decision: a `Pass` may stand in a function. It becomes a pass
  of the calling shader, so a wide blur can be a function.
- A function calling itself, also through another, says so at the
  node and is not set in. The palette does not offer such a call.
- The Shader tab showed only the compiler's problems. A call nothing
  reaches was never compiled and looked healthy, so a call now also
  shows its own problem there, as in a behavior.

Tests:
- Gate: 228 files, 3510 tests, CHECK EXIT 0 (+13: 11 in
  tests/core/shader-functions.test.ts, 2 for the new icon)
- e2e 28: 100 checks, all passed (+3 in 28l-viii). Called and in
  place both #606060/#a2a2a2, halved in the function #303030/#515151.
- Counter-check: a fingerprint without the called functions fails 1
  unit test, and in the browser the change check (#606060 stays).
- The first e2e runs found two faults, both fixed: a double-click
  only selected the function, and an unreached call of itself
  showed no problem.
- e2e total 1728 is computed (1725 + 3).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/panels/assetGroups.ts

@@ -31,6 +31,7 @@ export const ASSET_GROUP_LABELS: Record<AssetType, string> = {
   tilemap: 'Tilemaps',
   statemachine: 'State machines',
   shader: 'Shaders',
+  shaderfunction: 'Shader functions',
   font: 'Fonts',
   sound: 'Sounds',
   particles: 'Particles',

src/editor/panels/assetIcons.ts

@@ -22,6 +22,7 @@ import plugins from './icons/plugins.svg?raw';
 import rigidbody from './icons/rigidbody.svg?raw';
 import scene from './icons/scene.svg?raw';
 import shader from './icons/shader.svg?raw';
+import shaderfunction from './icons/shaderfunction.svg?raw';
 import sound from './icons/sound.svg?raw';
 import sprite from './icons/sprite.svg?raw';
 import statemachine from './icons/statemachine.svg?raw';
@@ -50,6 +51,7 @@ export const ASSET_ICON_SVG: Record<AssetType, string> = {
   rigidbody,
   scene,
   shader,
+  shaderfunction,
   sound,
   sprite,
   statemachine,

src/editor/panels/icons/shaderfunction.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-8 -8 16 16" width="16" height="16">
+  <!-- The shader's half-lit disc between the brackets of a call: a part used in other shaders. -->
+  <circle cx="0" cy="0" r="3.6" fill="none" stroke="currentColor" stroke-width="1.25" />
+  <path d="M0 -3.6 A3.6 3.6 0 0 1 0 3.6 Z" fill="currentColor" />
+  <path d="M-4.6 -6.8 Q-7.6 0 -4.6 6.8 M4.6 -6.8 Q7.6 0 4.6 6.8" fill="none" stroke="currentColor" stroke-width="1.25" stroke-linecap="round" />
+</svg>