Media: Square and Circle gallery images squashed (#12732)

This patch fixes images getting squashed/skewed in Square and Circle Tiled Gallery as described in 7861-wpcom.

This bug currently only appears when **Classic Editor** is used to create Square or Circle Tiled Gallery on **Simple sites**.

It does not appear in Jetpack site donpark.wpsandbox.me with latest Jetpack. I don't see an equivalent fix in Jetpack so it's not clear why the same issue does not appear there.

This commit was generated from D28285-code.

modules/tiled-gallery/tiled-gallery/tiled-gallery.css

@@ -86,9 +86,16 @@
 
 .tiled-gallery.type-circle .tiled-gallery-item img {
 	border-radius: 50% !important; /* Ensure that circles are displayed in themes that add border-radius to all images as a default */
+	object-fit: cover;
 }
 .tiled-gallery.type-circle .tiled-gallery-caption {
 	display: none;
 	opacity: 0;
 }
 
+
+/* =Square Layout
+-------------------------------------------------------------- */
+.tiled-gallery.type-square .tiled-gallery-item img {
+	object-fit: cover;
+}