Save scroll state immediately before updating

Signed-off-by: Oliver Hunt <oliver@hunt.bz>

src/components/structures/ScrollPanel.js

@@ -160,6 +160,10 @@ module.exports = React.createClass({
         this.checkFillState();
     },
 
+    componentWillUpdate: function(nextProps, nextState) {
+        this._saveScrollState();
+    },
+
     componentDidUpdate: function() {
         // after adding event tiles, we may need to tweak the scroll (either to
         // keep at the bottom of the timeline, or to maintain the view after