fix: add GLSLShader to canvas image preview node types (#9198)

## Summary

Add `GLSLShader` to `CANVAS_IMAGE_PREVIEW_NODE_TYPES` so GLSL shader
previews are promoted through subgraph nodes.

## Changes

- Add `'GLSLShader'` to the `CANVAS_IMAGE_PREVIEW_NODE_TYPES` set in
`src/composables/node/useNodeCanvasImagePreview.ts`

## Context

GLSLShader node previews were not showing on parent subgraph nodes
because `CANVAS_IMAGE_PREVIEW_NODE_TYPES` only included `PreviewImage`
and `SaveImage`. The `$$canvas-image-preview` pseudo-widget was never
created for GLSLShader nodes, so the promotion system had nothing to
promote. This degraded the UX of all 12 shipped GLSL blueprint subgraphs
— users couldn't see shader output previews without expanding the
subgraph.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9198-fix-add-GLSLShader-to-canvas-image-preview-node-types-3126d73d3650817dbe9beab4bdeaa414)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/widgets/BaseWidget.ts

@@ -27,6 +27,8 @@ export interface DrawWidgetOptions {
   showText?: boolean
   /** When true, suppresses the promoted outline color (e.g. for projected copies on SubgraphNode). */
   suppressPromotedOutline?: boolean
+  /** Transient image source for preview widgets rendered on behalf of another node (e.g. subgraph promotion). */
+  previewImages?: HTMLImageElement[]
 }
 
 interface DrawTruncatingTextOptions extends DrawWidgetOptions {