OO-725 fix overlapping peekview previews

src/main/java/org/olat/course/nodes/video/_content/peekview.html

+<div class="o_video_peekview">
+	<div class="o_video_poster" style="background-image: url('$mediaUrl/poster.jpg');"></div>
+</div>
 
-	<img src="$mediaUrl/poster.jpg" alt="poster" />

src/main/webapp/static/themes/light/modules/_video.scss

@@ -41,6 +41,10 @@
 	}
 }
 
+.o_video_peekview {
+	text-align: center;
+}
+
 .o_video_listing {
 	margin-right: -10px;