Fix focal point cropping in MediaGallery, fix focal point modal (#6740)

* Use object-position with object-fit instead of JS top/left

* Fix focal point modal

app/javascript/mastodon/features/ui/components/focal_point_modal.js

@@ -103,7 +103,7 @@ export default class FocalPointModal extends ImmutablePureComponent {
     const height = media.getIn(['meta', 'original', 'height']) || null;
 
     return (
-      <div className='modal-root__modal video-modal'>
+      <div className='modal-root__modal video-modal focal-point-modal'>
         <div className={classNames('focal-point', { dragging })} ref={this.setRef}>
           <ImageLoader
             previewSrc={media.get('preview_url')}