Fix perf display container background (#477)

Author: beaufortfrancois

sample/computeBoids/main.ts

@@ -15,7 +15,7 @@ quitIfWebGPUNotAvailable(adapter, device);
 
 const perfDisplayContainer = document.createElement('div');
 perfDisplayContainer.style.color = 'white';
-perfDisplayContainer.style.backdropFilter = 'blur(10px)';
+perfDisplayContainer.style.background = 'black';
 perfDisplayContainer.style.position = 'absolute';
 perfDisplayContainer.style.bottom = '10px';
 perfDisplayContainer.style.left = '10px';