Fix a bug where Webcam video was overflowing its container

packages/@uppy/webcam/src/style.scss

@@ -16,6 +16,7 @@
   overflow: hidden;
   background-color: $color-black;
   text-align: center;
+  position: relative;
 }
 
   .uppy-size--md .uppy-Webcam-videoContainer {
@@ -27,6 +28,12 @@
   // height: 100%;
   max-width: 100%;
   max-height: 100%;
+  position: absolute;
+  top: 0;
+  right: 0;
+  bottom: 0;
+  left: 0;
+  margin: auto;
 }
 
   .uppy-Webcam-video--mirrored {