Set premultipliedAlpha to false in webgpu

src/Engines/thinEngine.ts

@@ -515,7 +515,7 @@ export class ThinEngine {
     /**
      * Defines whether the engine has been created with the premultipliedAlpha option on or not.
      */
-    public readonly premultipliedAlpha: boolean = true;
+    public premultipliedAlpha: boolean = true;
 
     /**
      * Observable event triggered before each texture is initialized

src/Engines/webgpuEngine.ts

@@ -290,6 +290,7 @@ export class WebGPUEngine extends Engine {
 
         this._canvas = canvas;
         this._options = options;
+        this.premultipliedAlpha = false;
 
         this._hardwareScalingLevel = 1;
         this._mainPassSampleCount = options.antialiasing ? this._defaultSampleCount : 1;