Use OffscreenCanvas as intended for all code-paths in `src/display/…

…text_layer.js` (PR 15722 follow-up)

Currently some `getCtx` calls will have `isOffscreenCanvasSupported === undefined` set, meaning that `OffscreenCanvas` isn't being used as intended, since no `TextLayerRenderTask._isOffscreenCanvasSupported` property exists.

*Please note:* This patch is written using the GitHub UI, since I'm currently without a dev machine, so hopefully it works correctly.

src/display/text_layer.js

@@ -311,6 +311,7 @@ class TextLayerRenderTask {
     this._container = this._rootContainer = container;
     this._textDivs = textDivs || [];
     this._textContentItemsStr = textContentItemsStr || [];
+    this._isOffscreenCanvasSupported = isOffscreenCanvasSupported;
     this._fontInspectorEnabled = !!globalThis.FontInspector?.enabled;
 
     this._reader = null;