Make browserWindow of splashScreen transparent (#13699)

Currently an image with a transparent background is rendered with a white background.
This looks weird and is not the expected behavior when a transparent image is used.
With this change the browserWindow is set to transparent, so that the background is not rendered.
If adoptors want a white background they can simply use an image with a white background (or any other color).

packages/core/src/electron-main/electron-main-application.ts

@@ -346,7 +346,8 @@ export class ElectronMainApplication {
             ...splashScreenBounds,
             frame: false,
             alwaysOnTop: true,
-            show: false
+            show: false,
+            transparent: true,
         });
 
         if (this.isShowWindowEarly()) {