Implement slightly hacky CSS soln. to thumbnail sizing

As the slightly nicer alternative to fixupHeight being applied once
we actually have a timelineWidth.

The niceness comes from not needing timelineWidth, which means we can
implement at render time with CSS. (Despite still calculating aspect
ratios when we render.)

res/css/views/messages/_MImageBody.scss

@@ -20,5 +20,14 @@ limitations under the License.
 }
 
 .mx_MImageBody_thumbnail {
-    max-width: 100%;
-}
+    position: absolute;
+    width: 100%;
+    height: 100%;
+    left: 0;
+    top: 0;
+}
+
+.mx_MImageBody_thumbnail_container {
+    overflow: hidden;
+    position: relative;
+}