Force presentation canvas to white background, to properly show files which have no background.

html/sandboxes/odfcanvas_sandbox.html

@@ -28,6 +28,7 @@
 			user-select:none;
 			-webkit-user-select:none;
 			-moz-user-select:none;
+			background:white;
 		}
 		</style>
 	</head>

html/sandboxes/pdfcanvas_sandbox.html

@@ -22,6 +22,7 @@
 			margin:0 auto;
 			position:relative;
 			display:none;
+			background:white;
 		}
 		</style>
 	</head>