Clear the dimensions cache after updating the soft wrap column

Updating the soft wrap column could cause us to compute different values
for derived dimensions, so any dimensions that were cached *in the
process* of updating the soft wrap column need to be cleared.

src/text-editor-component.js

@@ -2118,6 +2118,7 @@ class TextEditorComponent {
       // rendered start row accurately. 😥
       this.populateVisibleRowRange(renderedStartRow)
       this.props.model.setEditorWidthInChars(this.getScrollContainerClientWidthInBaseCharacters())
+      this.derivedDimensionsCache = {}
 
       this.suppressUpdates = false
     }