Fix a scrolling glitch when the editor is updated before first measure

FIX: Fix an issue where dispatching an update to an editor before it measured itself
for the first time could cause the scroll position to incorrectly move.

Closes codemirror/dev#1366

src/viewstate.ts

@@ -114,7 +114,7 @@ export class ViewState {
   editorHeight = 0 // scrollDOM.clientHeight, unscaled
   editorWidth = 0 // scrollDOM.clientWidth, unscaled
   scrollTop = 0 // Last seen scrollDOM.scrollTop, scaled
-  scrolledToBottom = true
+  scrolledToBottom = false
   // The CSS-transformation scale of the editor (transformed size /
   // concrete size)
   scaleX = 1