Ask the driver what filter a texture actually got

Chasing the flaky pixel-art check needs to tell two things apart that
look identical in a screenshot: a scale mode the model never changed,
and one that changed but never reached the sampler. Only the driver can
answer the second, so `__gm.textureFilter(file)` returns the model's
scaleMode next to `gl.getTexParameter` on the uploaded texture.

It binds through `renderer.texture.bindSource` rather than a raw
`bindTexture`, so measuring does not corrupt Pixi's idea of what is
bound and become a source of flakiness itself. `TextureLibrary.peek`
looks a picture up without starting a load or touching its scale mode,
which `get` would do.

Sixty measurements later the filter path is cleared: model and driver
agreed every time, across repeated switches, after the full module
run-up, and through a forced context loss. The instrument stays; the
note in todo-notes/flaky-pixel-art-filter.md stays open.

src/viewport/TextureLibrary.ts

@@ -42,6 +42,11 @@ export class TextureLibrary {
     return texture;
   }
 
+  /** The picture if it is in memory, without starting a load or touching the scale mode. */
+  peek(file: string): Texture | null {
+    return this.loaded.get(file) ?? null;
+  }
+
   /** Runs `fn` whenever a picture has arrived; returns the unsubscribe. */
   onChange(fn: () => void): () => void {
     this.listeners.add(fn);