Rework DOM measurement to try to prevent measurement errors

* Simplify scrollbar refresh and measurement by using imperative DOM
  manipulation instead of React to hide/show scrollbars.
* Rename `::performInitialMeasurement` to `::becameVisible`
* Break `::checkForVisibilityChange` out of `::pollDOM` and use it in
  to check for the element becoming visible in `componentWillUpdate`.
* Don't rely on stored visibility state anywhere. Always check again.
  This could potentially be cached for an update cycle but being wrong
  about this is disastrous so I'm being conservative.

src/display-buffer.coffee

@@ -31,11 +31,11 @@ class DisplayBuffer extends Model
     scrollTop: 0
     scrollLeft: 0
     scrollWidth: 0
+    verticalScrollbarWidth: 15
+    horizontalScrollbarHeight: 15
 
   verticalScrollMargin: 2
   horizontalScrollMargin: 6
-  horizontalScrollbarHeight: 15
-  verticalScrollbarWidth: 15
   scopedCharacterWidthsChangeCount: 0
 
   constructor: ({tabLength, @editorWidthInChars, @tokenizedBuffer, buffer, @invisibles}={}) ->