constrain gif backgrounds (with modifications) (#176)

* constrain gif backgrounds. fixes #159

* nit: move background-* styles to AutoplayVideo.html

routes/_components/AutoplayVideo.html

@@ -1,7 +1,7 @@
 <video
-  class="{{className || ''}}"
+  class="autoplay-video {{className || ''}}"
   aria-label="{{ariaLabel || ''}}"
-  style="background: url({{poster}});"
+  style="background-image: url({{poster}});"
   :poster
   :width
   :height
@@ -11,4 +11,11 @@
   loop
   webkit-playsinline
   playsinline
-/>
+/>
+<style>
+  .autoplay-video {
+    background-repeat: no-repeat;
+    background-position: center;
+    background-size: contain;
+  }
+</style>